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Light Topic A Reflection Of light At Spherical Mirror Synopsis

WBBSE Class 10 Light Overview

1. If any reflecting surface is a part of a hollow sphere, then it is called a spherical mirror. A spherical mirror is of two types:

1. Concave mirror and
2. Convex mirror

2. A spherical mirror is called a concave mirror when its concave surface acts as its reflecting surface.

3. A spherical mirror is called a convex mirror when its convex surface gets as its reflecting surface.

4. The centre of curvature of a spherical mirror is the centre of the sphere of which the mirror is a part.

5. The radius of the sphere of which this mirror is a part, is called the radius of curvature of the spherical mirror.

6. The straight line passing through the pole of the spherical mirror and the centre of curvature of the mirror is called the principal axis of the mirror.

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7. In a spherical mirror, those rays which fall near the pole and make a small angle with the principal axis are called paraxial rays,

8. A beam of rays parallel to the principal axis of a concave mirror with a small aperture after reflection from the mirror converges to a definite point on the principal axis which is called the principal focus of the concave mirror.

9. A beam of rays parallel to the principal axis of a convex mirror with a small aperture after reflection from it appears to diverge from a fixed point on the principal axis, which is called the principal focus of the convex mirror.

10. A plane surface perpendicular to the principal axis and passing through the principal focus of a spherical mirror is called its focal plane.

11. If the focal length and radius of curvature of a spherical mirror are / and r respectively, then \(\).

12. Ray tracing method: The position, nature and size of the image of an extended object, formed by a spherical mirror can be determined geometrically. Any two of the following rays intersecting at a point will indicate the portion of the image—

1. A ray parallel to the principal axis, after reflection passes through the focus or appears to diverge from the focus.
2. A ray passing through the focus, after reflection emerges parallel to the principal axis.
3. A ray passing through the centre of curvature after reflection retraces its path in the opposite direction.

12. Characteristic of real image:

It is formed on the same side of the mirror as the object.

It is always inverted.

The size of the real image can be smaller or equal or larger than that of the object.

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13. Characteristics of virtual image:

It is always formed on the opposite side of the mirror as the object.

It is always erect,

The size of the virtual image becomes larger than the object or equal to it is in the case of a concave mirror. Whereas in case of a convex mirror, it is smaller than the object or equal to it.

14. Sign convention for a spherical mirror: Cartesian sign convention:

All distances are to be measured from the pole of the spherical mirror.

All distances measured in the same direction as that of incident rays are to be taken as positive and all distances measured in a direction opposite to that of the incident rays are to be taken as negative.

If the principal axis of the mirror is taken as X-axis, the upward distance along +ve Y- axis is taken as positive while the downward distance along the -ve Y-axis is taken as negative.

Light Topic A Reflection Of Light At Spherical Mirror Short And Long Answer Type Questions

Understanding Reflection and Refraction of Light

Question 1. What are the focal length and focal plane of a spherical mirror? Show them with appropriate figures.

Answer:

The focal length and focal plane of a spherical mirror

Focal length

h: The distance between the pole and the principal focus of a spherical mirror is called its focal length.
Focal plane: A plane surface perpendicular to the principal axis and passing through the principal focus of a spherical mirror is called its focal plane.

 

MPM₁ is the principal section of both the concave and the convex mirrors, where P is the pole and F is the principal focus. PF is the focal length and ABCD is the focal plane.

Wbbse Class 10 Physical Science Solutions

Question 2. Establish a relationship between focal distance and radius of curvature of a concave mirror for paraxial rays.

Answer:

A relationship between focal distance and radius of curvature of a concave mirror for paraxial rays

MPM₁ is the principal section of a concave mirror with a small aperture. P, F and C are pole, principal focus and centre of curvature respectively of this concave mirror.

Focal length, PF = ƒ and radius of curvature, PC = r. A ray of light AB parallel to the principal axis of the concave mirror falls at point B and passes through the focus (F) after reflection. C and B are joined by a straight line. CB is normal on the mirror at point B.

According to the figure, angle of incidence, ∠ABC = angle of reflection, ∠FBC.
Again, AB || CP and BC is a transversal.

∴ ∠ABC = ∠BCF [alternate angles]
∴ ∠BCF = ∠FBC
∴ CF=FB of the ΔBCF

As the incident ray is paraxial, B and P are very close to each other.
In that case, FB ≈ PF, i.e., PF = CF PC  or, PF = PC/2    or, ƒ = r/2 [∵ pf = ƒ and PC = r]

 

Laws of Reflection Explained

Question 3. Establish a relationship between focal distance and radius of curvature of a convex mirror for paraxial rays.

Answer:

A relationship between focal distance and radius of curvature of a convex mirror for paraxial ray

MPM₁ is the principal section of a convex mirror with a small aperture. P, F and C are the poles, principal focus and centre of curvature respectively of this convex mirror.

∴ focal length, PF = ƒ and radius of curvature, PC = r.

A ray AB, parallel to the principal axis of the convex mirror, falls at point B and is reflected along the path BD. If BD is extended backwards, it intersects principal focus at points F, C and B are joined by a straight line and extended up to E. CF is normal on the mirror at point B.

From the angle of incidence, ∠ABE = angle of reflection, ∠DBE

Again, AB || PC and EC are the transversal.
∴ ∠ABE = ∠FCB [corresponding angle]

Again, ∠FBC =  ∠DBE [vertically opposite angles]
∴ ∠FBC = ∠FCB
∴ FB = FC of the ΔBCF

If the incident rays are paraxial, points B and P are very close to each other.

In that case, FB ≈ PF or, PF = FC or, PF = PC/2 or, PF = FC

 

 

Question 4. what can you see if you place your face in front of the concave and convex sides of a shining spoon?

Answer: If one keeps his face near the concave side of the shining spoon, a magnified erect image can be seen in the spoon. If the face is now placed at a large distance from the spoon, the image becomes inverted and diminished. On the other hand, if one places his face in front of the convex side of the spoon, an erect and diminished image will be seen for all distances of the face from the spoon.

 

 

 

Question 5. What is a spherical mirror? How many types of spherical mirrors are there? Mention them.

Answer:

A spherical mirror:

A spherical mirror is a reflecting surface which is a part of a hollow sphere.

There are two types of spherical mirrors:
(1) Concave spherical mirror
(2) Convex spherical mirror

Wbbse Class 10 Physical Science Solutions

Question 6. Define a convex mirror and a concave mirror.

Answer:
Convex mirror: A spherical mirror, whose bulging or convex surface is used for reflection is called a convex mirror.
Concave mirror: A spherical mirror whose concave surface is used for reflection is called a concave mirror.

Question 7. Define the pole and principal axis of a spherical mirror; Show them with appropriate figures.

Answer:
Pole: The central point P of the aperture of the spherical mirror is called its pole.

Applications of Light in Daily Life

Principal axis: The principal axis of a spherical mirror is a straight line XP obtained by connecting the centre of curvature and the pole of the spherical mirror.

Question 8. Define the centre of curvature and radius of curvature of a spherical mirror. Show them with the appropriate Image.

Answer:
Center of curvature: The centre of curvature of a spherical mirror is the centre of the hollow sphere of which the mirror is a part. In Image, C is the centre of curvature.

The radius of curvature: The radius of the hollow sphere of which this spherical mirror is a part, is called the radius of curvature of the spherical mirror. In the Image, PC is the radius of curvature.

Question 9. What are paraxial rays?

Answer:

Paraxial rays

Paraxial rays are those rays which fall very close to the pole of the spherical mirror and make very small inclination with the principal axis.

Wbbse Class 10 Physical Science Solutions

MPM₁ is the principal section of a concave mirror whose pole is P. The two rays AB and CD fall near the pole and the inclination of these two rays with the principal axis is small. So AB and CD are called paraxial rays.

 

Question 10. What is the principal focus of a concave mirror and a convex mirror?

Answer:

The principal focus of a convex mirror: A beam of rays parallel to the principal axis of a convex mirror with a small aperture, after reflection from it, appears to diverge from a fixed point on it principal axis. This fixed point is called the principal focus of the convex mirror. In image, F is the principal focus of the convex mirror.

 

 

 

The principal focus of a convex mirror: A beam of rays parallel to the principal axis of a convex mirror with a small aperture, after reflection from its, appears to diverge from a fixed point on it principal axis. This fixed point is called the principal focus of the convex mirror. In image, F is the principal focus of convex mirror.

 

 

Question 11. What is the secondary: focus of a spherical mirror?

Answer:

The secondary: focus of a spherical mirror:

A parallel beam of rays inclined with the principal axis, is incident on a spherical mirror. In case of a concave mirror, the reflected rays meet at a point on its focal plane. On the other hand, if the mirror is a convex one, the reflected rays appear to diverge from a point on the focal plane. In each case the said point on the focal plane is called the secondary focus of the mirror used.

MPM₁ is the principal section of both the concave as well as the convex mirror, where P is the pole, F is the principal focus, ABCD is the focal plane and F’ is the secondary focus.

 

 

Question 12. Draw a clear sketch of the formation of images of a point object in a concave mirror under the given conditions. Mention the position of the image and its nature in each of the cases.
(1)The distance of the object from the mirror is more than the radius of curvature
(2)The object is placed at the centre of curvature.

Answer:

 

 

 

Location of object	Location of image	Nature of image
1. The distance of the object from the mirror (PR) is more than the radius of curvature (PC)	Between focus (P) and centre of curvature (C) [in the position R’]	Real image
2. The object is placed at the centre of curvature (C)	Image is formed at the centre of curvature (C)	Real image

 

Wbbse Class 10 Physical Science Solutions

Question 13. Draw a clear sketch of the formation of images of a point object in a concave mirror under the given conditions. Mention the position of image and its nature in each of the following cases,
(1)The object is placed between the centre of curvature (C) and the focus (F)
(2)The object is placed between the focus and the pole

Answer:

 

 

Location of object	Location of image	Nature of image
1. The object is placed between the centre of curvature (C) and the focus (F)	The image distance (PR ) from the mirror is greater than the radius of curvature (PC)	Real image
2. The object is placed between the focus (F) and the pole (F)	It is located at a point R’ behind the mirror	Virtual image

 

Question 14. Draw the ray diagram on the reflection of convergent rays by a concave mirror.

Answer: Here convergent rays are incident on a concave mirror. In absence of the mirror, the rays would have met at point O. After reflection from the mirror, the reflected rays meet at a point I on the principal axis. Here, O is the virtual object and I is its real image. The convergent rays remain convergent even after reflection.

 

 

Question 15. Show the image formation of a point object in a convex mirror with a suitable figure.

Answer: Diverging light rays coming from a point object O, placed on the principal axis of a convex mirror become more diverging after reflection by the mirror and appear to diverge from a point I behind the mirror. I is the virtual image of point object O.

 

 

Question 16. Draw the ray diagram for each of a doncave mirror and a convex mirror under the following condition: The incident ray is parallel to the principal axis.

Answer: Charactaristic of incident ray: It is parallel to the principal axis.

 

 

Characteristic of reflected ray:
In case of a concave mirror: Reflected ray is directed towards the focal point.
In case of a convex mirror: If the reflected ray is extended backwards, it passes through the focal point.

Question 17. Draw the ray diagram for each of the concave and a convex mirrors under the following condition: The incident ray is directed towards the focal point.

Answer:

Characteristic of incident ray: It is directed towards the focal point (F).
Characteristic of reflected ray: It is parallel to the principal axis for both mirrors.

Wb Class 10 Physical Science Question and Answers

Question 18. Draw the ray diagram for each of a concave mirror and a convex mirror under the following condition: The incident ray is directed towards the centre of curvature of the mirror.

Answer:

Characteristic of incident ray: It is directed towards the centre of curvature (C) of the mirror.
Characteristic of reflected ray: It retraces the path of the incident ray in both the cases.

Question 19. Draw clear sketches of the formation of images of an extended object in a concave mirror under the following conditions. Mention the position of image, its size and also its nature in each case.

(1)Object is placed at infinity
(2)Object is not at infinity but beyond the centre of curvature
(3)Object is placed at the centre of curvature
(4)Object is placed between centre of curvature and focus
(5)Object is placed at the focus
(6)Object is placed between focus and pole

Answer:

Location of object	Location of image	Size of image	Nature of image
1. At infinity	Focus	Very small compared to the object, almost like a point	Real, inverted
2. Between  infinity and centre of curvature C	Between focus F and centre of curvature C	Small compared to the object	Real, inverted
3. Centre of curvature C	Centre of curvature C	Equal to the object	Real, inverted
4. Between the centre of curvature C and focus F	Between the centre of curvature C and infinity	Large compared to the object (magnified)	Real, inverted
5. At focus F	Infinity	Very large compared to the object, nearly infinite	Real, inverted
6. Between focus F and pole D	Behind the mirror between pole P and infinity	Larger than the object	Virtual, erect

 

 

 

Question 20. Draw the ray diagram for each of a concave and a convex mirror under the following condition: The incident ray is directed towards the pole of the mirror.

Answer:

 

Characteristic of incident ray: It is directed towards the pole of the mirror.
Characteristic of reflected ray: Both the incident ray and the reflected ray are inclined to the principal axis at the same angle.

Question 21. Write down some applications of a convex mirror.

Answer:

Some applications of a convex mirror

(1)Convex mirrors are used in street lamps as reflectors. This mirror diverges the light of a street lamp over a very large area.
(2)Convex mirrors are used in motorcycles, buses, lorries etc. as viewfinders. A convex mirror forms a virtual, erect but diminished image of an object kept in front of it. As a result, the driver of a vehicle can see the diminished, virtual image of another vehicle, object or individual far away from him and can drive his vehicle without causing any accident.

Question 22. Write down some applications of a concave mirror.

Answer:

Some applications of a concave mirror

1. A polished metal surface which works like a concave mirror is used in torches, searchlights or headlights of cars. In these three, the bulb is placed at the focus of the mirror so that the rays emitted from the bulb are reflected by the concave reflector and become parallel. This is very convenient for the driver as he can see up to a large distance.

 

 

 

2. Concave mirrors are used by dentists This mirror forms a magnified and erect image of teeth so that any cavity or crack in the teeth can be seen very clearly.
3. If any object is placed within the focus (i.e., between the pole and the focus) of a concave mirror, a magnified virtual image is formed. This property is utilised while using a concave mirror as a shaving mirror or in a beauty parlour.

Question 23. You are given three mirrors—one plane, one convex and the last one concave. How would you identify them without touching them?

Answer: A plane mirror always produces a virtual image of the same size as that of the object. A convex mirror always produces a diminished virtual image of the object and for a concave mirror, a magnified virtual image is produced if the object is placed at a distance less than the focal length.

Suppose, a pencil is held very close to a mirror. If we get an image of the same size as that of the object, then the mirror is a plane mirror.
If a diminished image is formed, then the mirror is convex and if a magnified image is formed, then the mirror is concave.

Question 24. What Is the change in its focal length if a spherical mirror is immersed in water?

Answer:

Change in its focal length if a spherical mirror is immersed in water

Light is reflected from a spherical mirror and the laws related to the reflection of light do not depend on the nature of the medium surrounding the mirror. So if a spherical mirror is immersed in water, there is no change in focal length.

 

Light Topic A Reflection Of Light At Spherical Mirror Very Short Answer Type Questions Choose The Correct Answer

 

Question 1. What is the radius of curvature of a spherical mirror whose focal lenght is ƒ?

1. 2ƒ
2. ƒ
3. 3
4. ƒ/2

Answer: 1. 2ƒ

Question 2. What is the focal length of a concave mirror whose radius of curvature is 20 cm?

1. 20cm
2. 15cm
3. 10 cm
4. 40 cm

Answer: 2. 15cm

Question 3. If an object is placed between the pole and the principal focus of a concave mirror, the image will be

1. Real/magnified
2. Virtual, diminished
3. Real, diminished
4. Virtual, magnified

Answer: 4. Virtual, magnified

Question 4. If an object is placed beyond the centre of curvature of a concave mirror, the image will be

1. Real, magnified
2. Virtual, diminished
3. Real, diminished
4. Virtual, diminished

Answer: 3. Real, diminished

Question 5. If an object is kept at the centre of curvature of a concave mirror, the image will be

1. Real, of the same size
2. Virtual, diminished
3. Real, diminished
4. Virtual, magnified

Answer: 1. Real, of the same size

Question 6. If an object is kept between the focus and the centre of curvature of a concave mirror, the image will be

1. Real, magnified
2. Virtual, diminished
3. Real, diminished
4. Virtual, magnified

Answer: 1. Real, magnified

Question 7. If an object is kept in front of a convex mirror, the image will be

1. Virtual, magnified
2. Virtual, diminished
3. Virtual, of the same size
4. Real, diminished

Answer: 2. Virtual, diminished

Question 8. The following is used as the viewfinder of an automobile

1. Concave mirror
2. Plane mirror
3. Convex mirror
4. None of the above

Answer: 3. Convex mirror

Question 9. A small aperture for a spherical mirror means an aperture of

1. Less than 10°
2. Less than 5°
3. Less than 15°
4. Less than 20°

Answer: 1. Less than 10°

Question 10. A ray, parallel to the principal axis of a concave mirror, after reflection

1. Passes through the centre of curvature
2. Retraces its path
3. Passes through the principal focus
4. Passes through the pole of the mirror

Answer: 3. Passes through the principal focus

Question 11. In the Image Paraxial rays are

 

1. AB
2. CD
3. EF
4. AB, CD, EF

Answer: 1. AB

Question 12. A ray, parallel to the principal axis of a convex mirror, after reflection—

1. Passes through the centre of curvature
2. Appears to come from the centre of curvature
3. Passes through the principal focus
4. Appears to come from the principal focus

Answer: 4. Appears to come from the principal focus

Question 13. A ray, directed toward principal focus of a concave mirror, after reflection

1. Returns back following the same path
2. Passes parallel to the principal axis
3. Passes through the centre of curvature
4. Passes along the principal axis

Answer: 2. Passes parallel to the principal axis

Question 14. A ray, directed towards the principal focus of a convex mirror, after reflection

1. Returns back following the same path
2. Passes parallel to the principal axis
3. Passes through the centre of curvature
4. Appears to come from the centre of curvature

Answer: 2. Passes parallel to the principal axis

Question 15. A ray, directed towards the centre of curvature of a concave mirror, after reflection

1. Returns back following the same path
2. Passes parallel to the principal axis
3. Passes through the principal focus
4. Passes along the principal axis

Answer: 1. Returns back following the same path

Question 16. A ray, directed towards the centre of curvature of a convex mirror, after reflection

1. Returns back following the same path
2. Passes parallel to the principal axis
3. Appears to come from the principal focus
4. Passes through the principal focus

Answer: 1. Returns back following the same path

Question 17. Number of principal focus of a spherical mirror

1. One
2. Two
3. Three
4. Four

Answer: 1. One

Question 18. While using an electric bulb, the type of mirror used as the reflector is

1. Concave
2. Convex
3. Plane
4. Parabolic

Answer: 2. Convex

Question 19. The type of mirror used by dentists for examination of the teeth of a person is

1. Concave
2. Convex
3. Plane
4. Parabolic

Answer: 1. Concave

Question 20. The type of mirror used as a saving mirror is

1. Convex
2. Concave
3. Plane
4. Parabolic

Answer: 2. Concave

Question 21. The angle of incidence for a ray of light passing through the centre of curvature of a concave mirror is

1. 0°
2. 45°
3. 90°
4. 180°

Answer: 1. 0°

Question 22. The angle of deviation of a ray of light incident on a concave mirror along the principal axis is

1. 0°
2. 90°
3. 180°
4. 360°

Answer: 3. 180°

Light Topic A Reflection Of Light At Spherical Mirror Answer In Brief

 

Question 1. When does a concave mirror form a virtual image of an object?
Answer: If an object is placed between the pole and the focus of a concave mirror, a virtual image of the object is formed.

Question 2. When does a concave mirror form a real image of an object?
Answer: If an object is placed away from the focus of the concave mirror or the object distance is more than the focal length, a real image of the object is formed.

Question 3. If an object is placed at a distance of more than 2ƒ in front of a concave mirror, where is the image formed?
Answer: The image is formed in front of the mirror, between the focus and the centre of curvature.

Question 4. If an object is placed at the centre of curvature of a concave mirror, where is the Image formed?
Answer: The image is formed too at the centre of curvature.

Question 5. If an object is placed between the focus and the centre of curvature, in front of a concave mirror, where is the image formed?
Answer: The image is formed in front of the mirror at a distance more than 2ƒ from the mirror.

Question 6. State whether the virtual image of an object formed by a concave mirror is magnified or diminished.
Answer: The virtual image of an object formed by a concave mirror is magnified.

Question 7. State whether the image of an object formed by a convex mirror is real or virtual.
Answer: The image of an object formed by a convex mirror is always virtual.

Question 8. State whether the virtual image of an object formed by a convex mirror is magnified or diminished.
Answer: The virtual image of an object formed by a convex mirror is diminished.

Question 9. State whether the centre of curvature of a concave mirror is situated in front of the reflecting surface or behind it.
Answer: The centre of curvature of a concave mirror is situated in front of it.

Question 10. State whether the centre of curvature of a convex mirror is located in front of the reflecting surface or behind it.
Answer: The centre of curvature of a convex mirror is located behind the reflecting surface.

Question 11. State whether the principal focus of a concave mirror is located in front of the reflecting surface or behind if.
Answer: The principal focus of a concave mirror is located in front of the reflecting surface.

Question 12. State whether the principal focus of a convex mirror is located in front of the reflecting surface or behind it.
Answer: The principal focus of a convex mirror is located behind the reflecting surface.

Question 13. How many principal focus points are there for a spherical mirror?
Answer: There is only one principal focus point for a spherical mirror.

Question 14. How many secondary focus points are there for a spherical mirror?
Answer: There are innumerable secondary focus points for a spherical mirror.

Question 15. What is the path of the reflected ray if the incident ray is directed towards the centre of curvature of a spherical mirror?
Answer: If the incident ray is directed towards the centre of curvature of a spherical mirror, the reflected ray retraces back the path of the incident ray.

Question 16. What is the focal length of a spherical mirror with a small aperture, which has a radius of curvature of 40 cm?
Answer: The focal length of a spherical mirror with a small aperture, which has a radius of curvature of 40 cm is 40/2 or 20 cm.

Question 17. What is the nature of a spherical mirror if it always produces virtual images?
Answer: If the spherical mirror always produces a virtual image, the mirror is convex in nature.

Question 18. What is the focal length of a spherical mirror with a small aperture, when the distance between the focus and the centre of curvature of the mirror is 20 cm?
Answer: The focal length of a spherical mirror is 20cm when the distance between the focus and the centre of curvature of the mirror is 20cm.

Question 19. A spherical mirror forms an image of the same size as that of the object for a particular position of the object. What is the nature of the mirror?
Answer: The spherical mirror is concave in nature.

Question 20. Is it possible for a mirror to form a virtual image which is smaller than the object?
Answer: Yes, a virtual image of an object formed by a convex mirror is smaller than the object.

Question 21. What is the nature of the reflected beam of light when a parallel beam of light is incident on a concave mirror?
Answer: When a parallel beam of light is incident on a concave mirror, the reflected beam of light is converging in nature.

Question 22. What is the nature of the reflected beam of light when a parallel beam of light is incident on a convex mirror?
Answer: When a beam of parallel light is incident on a convex mirror, the beam of reflected light is diverging in nature.

Question 23. What type of mirror is used as the reflector in a street lamp?
Answer: A convex mirror is used as the reflector in a street lamp.

Question 24. State whether the image of an object placed at the centre of curvature of a concave mirror is diminished or magnified.
Answer: The image of the object is neither diminished nor magnified it is of the same size as that of the object.

Question 25. What is the value of the focal length of a plane mirror?
Answer: The focal length of a plane mirror is infinite.

Question 26. Is the virtual image of an object formed by a concave mirror, laterally inverted?
Answer: Yes, the virtual image of an object formed by a concave mirror is laterally inverted.

Question 27. Keeping the relative position of an object with respect to a spherical mirror unchanged, the entire arrangement is immersed in water. Will there be any change in the position of the image?
Answer: No, the position of the image with respect to the mirror will remain unchanged.

Question 28. What type of mirror is used In a reflecting telescope?
Answer: A concave mirror is used in a reflecting telescope.

Question 29. Among a plane, a concave and a convex mirror, which one has the largest field of view?
Answer: Among the three mirrors, a convex mirror has the largest field of view.

Question 30. What is the value of the radius of curvature of a plane mirror?
Answer: The radius of curvature of a plane mirror is infinitely large.

Question 31. The focal length of a concave mirror is 10 cm. What is its focal length when it is immersed in water?
Answer: When the concave mirror is immersed in water, its focal length remains unchanged. So, the focal length of the mirror remains 10 cm.

Question 32. What is the nature of the mirror of a dentist?
Answer: The mirror of a dentist is concave in nature.

Question 33. Which type of mirror is used in the headlight of an automobile?
Answer: A concave mirror is used in the headlight of an automobile.

Question 34. Which type of mirror is used in a solar cooker?
Answer: A concave mirror is used in a solar cooker.

Light Topic A Reflection Of Light At Spherical Mirror Fill In The Blanks

Question 1. The concave side of a shining spoon behaves like a _____ mirror.
Answer: Concave

Question 2. The convex side of a shining spoon behaves like a _______ mirror.
Answer: Convex

Question 3. The _____ focus of a spherical mirror is a fixed point.
Answer: Principal

Question 4. The radius of curvature of a spherical mirror is ______ of that of the focal length.
Answer: Double

Question 5. Between a concave mirror and a convex mirror, a ______ mirror is converging.
Answer: Concave

Question 6. Between a concave mirror and a convex mirror, a mirror ______ is diverging.
Answer: Convex

Question 7. The middle point of a spherical mirror is known as ______
Answer: Pole

Question 8. When an object is placed at the centre of curvature of a concave mirror, magnification of the image formed by the mirror is _____
Answer: Unity

Question 9. Only rays inclined at an angle less than about 10° to the principal axis is considered as ____ rays.
Answer: Paraxial

Question 10. _____ mirror is used as a rear-view mirror in vehicles.
Answer: Convex

Light Topic A Reflection Of Light At Spherical Mirror State Whether True Or False

 

Question 1. There is no deviation of the light rays of reflection in a glass slab.
Answer: True

Question 2. The image of an object placed at the centre of curvature of a concave mirror is formed at the centre of curvature itself.
Answer: True

Question 3. The distance between the pole and the focus of a spherical mirror is known as the- focal length.
Answer: True

Question 4. The image formed by a concave mirror is always magnified in comparison to the object.
Answer: False

Question 5. The image formed by a convex mirror is always diminished in comparison to the object.
Answer: True

Question 6. Paraxial rays make a small angle with the principal axis of a mirror.
Answer: True

Question 7. A beam of light parallel to the principal axis of a concave mirror converges to a fixed point.
Answer: True

Question 8. A beam of light parallel to the principal axis of a convex mirror converges to a fixed point.
Answer: False

Question 9. Image formed by a concave mirror is always diminished in size.
Answer: False

Question 10. Image of an object placed between pole and focus of a concave mirror is always inverted.
Answer: False

 

Light Topic A Reflection Of Light At Spherical Mirror Numerical Examples

 

1. Angle of incidence (i) = angle of reflection (r)
2. For spherical mirror of a small aperture the relation between the focal length of the mirror (ƒ) and it’s the radius of curvature (r) is ƒ = r/2

Question 1. The focal length of a concave mirror is 15 cm. At which places in front of the mirror/an object should be placed/ so that
(1)A magnified real image and
(2)A magnified virtual image are formed?

Answer:

Given

The focal length of a concave mirror, / = 15 cm
∴ r= 2ƒ = 2 x 15 = 30 cm

Suppose, the object distance from the mirror = u

(1)In order to get a magnified real image, ƒ < u < 2ƒ or, 15 cm < u < 30 cm is the condition.
(2)In order to get a magnified virtual image, u<ƒ or, u < 15 cm is the condition.

Characteristics of Light Waves

Question 2. What is the focal length of a convex mirror having radius of curvature of 20 m?

Answer: Radius of curvature of the convex mirror (r) = 20 m
∴ Focal length of the convex mirror is
(f) = r/2= 20/2 = 10 m

Question 3. When a concave mirror is placed in sunlight, the image of the sun is formed at a distance 15 cm in front of the mirror. Find the radius of curvature of the mirror.

Answer: The rays of light coming from the sun are considered to be parallel. So the image of the sun is produced at the focus of the concave mirror.
∴ The focal length of the concave mirror (ƒ) = 15 cm
∴ Radius of curvature of the mirror (r) = 2ƒ= 2 x 15 = 30 cm

 

Topic B Refraction of Light Synopsis

 

1. In a homogeneous medium, ray of light travels in a straight line.
2. When a ray of light traveling in one medium is incident obliquely on the surface of another medium, a part of light is reflected back Into the same medium and rest of the light is transmitted into the other medium in a direction different from its initial path.

3. When light passes from one transparent homogeneous medium to another transparent homogeneous medium the phenomenon of change in the direction of path of light is called refraction.
(1)When a ray of light travels from a rarer medium to a denser medium, it bends towards the normal, i.e., ∠i > ∠r and deviation of the ray δ = i- r.

(2)When the ray of light travels from a denser medium to rarer medium, it bends away from the normal, i.e„  i < r.
The derivation of the ray, = r -i

(3)If the ray is incident normally on the surface separating two media, it goes undeviated, i.e., i = r = 0.
The deviation of the ray, 8 – 7 – r = 0
Light travels with different speeds in different media. In vacuum speed of light is 3 x 10⁸ m/s.

4. Cause of refraction of light: When a ray of light passes from one medium to another medium, its direction (or path) changes (except normal incident i.e., i = 0) because of change in speed of light from one medium to another.

5. Laws of refraction:
(1)First law of refraction: The incident ray, the refracted ray and normal to the point of incident on the surface of separation of the two media lie in the same plane.

(2)Second law of refraction: The ratio of the sine of the angle of incident i to the sine of the angle of refraction r is constant for the pair of given medium and for a particular colour of light.

The constant is called the refractive index of the second medium with respect to the first medium, it is generally represented by µ or n.

\(\frac{\sin i}{\sin r}={ }_1 \mu_2\)

 

The above law is known as Snell’s Law.

(3)The refractive index of a medium defined with respect to vacuum is called the absolute refractive index of the medium.
(4)The absolute refractive index of a medium is defined as the ratio of the speed of light in vacuum to the speed of light in that medium.

i.e…, \(\mu=\frac{\text { speed of light in vacuum or air }(c)}{\text { speed of light in that medium }(v)}\)

 

If v₁ is the speed of light in medium 1 and v₂ is the speed of light in medium 2 then, \({ }_1 \mu_2=\frac{v_1}{v_2}=\frac{c / v_2}{c / v_1}=\frac{\mu_2}{\mu_1}\)

According to the principle of reversibility \({ }_1 \mu_2=\frac{1}{{ }_2 \mu_1}\)

6. Change in speed, direction and wavelength of light due to refraction:
(1)Due to change in speed of light in passing from one medium to other the direction of ray of light changes’except for ∠i = 0.
(2)When a ray of light passes from a rarer to a denser medium, the speed of light decreases while if it passes from a denser to a rarer medium the speed of light increases.
(3) Since the frequency of light depends on the source of light, so it does not change on refraction.
(4)In refraction of light from one medium to another medium, due to change in the speed of light, the wavelength of light also changes because its frequency remains unchanged.

7. Refraction of light through a rectangular glass slab: When a ray of light is incident obliquely on the surface of a rectangular glass slab it emerges from the opposite surface after passing through the slab. Then both the incident ray and the emergent ray are parallel to each other. The separation between the incident ray and the emergent ray is called lateral displacement.

The lateral displacement depends on—
1. The thickness of the slab,
2. The angle of incidence (For normal incidence i.e., ∠i= 0; ∠r = 0 and the lateral displacement is zero).
3. The refractive index of glass.

8. Refraction of light through a prism: A prism is a portion of transparent refracting medium bounded by three rectangular and two triangular faces.

The angle between the refracting faces is called the angle of the prism (A).
The two rectangular plane inclined surface through which the light passes are called the refracting surfaces.
The section of the prism perpendicular to the refracting edge is called principal section of prism. shows the principal section ABC of a prism.

∠BAG = A —The angle of the prism.

 

 

If i₁ be the angle of incident, r₁ be the angle of refraction, i₂ be the angle of emergence, r₂ be the angle of incident at the AC surface and δ be the angle of deviation, then, r₁ + r₂ = A,  i₁ + i₂ – A

Shows the variation of angle of deviation (δ) with the angle of incedence (i₁).

In the position of minimum deviation i₁ = i₂ = i (say) and  δ=δ_m. Then, δ_m = 2i = – A.

9. For a given prism and a given colour of light/ 6m is unique.

Angle of deviation depends on the—
(1)The angle of incidence
(2) the angle of the prism
(3)The material of the prism
(4)The wavelength of the light used.

At minimum deviation position:
(1)Angle of incidence (i₁) = angle of emergence (i₂)
(2)Angle of refraction (r₁) = angle of incident at AC surface (r₂)
(3) The refracted ray inside a prism is parallel to the base of the prism.

 

Topic B Refraction of Light Short And Long Answer Type Questions

 

Question 1. Show the refraction of light in a prism with a ray diagram. What is the assumption made while drawing this diagram, regarding optical densities of the material of the prism and the surrounding medium?

Answer:

Description of figure:
ABC → Principal section of the prism
PQ → Incident ray
RS → Emergent ray
MQT → Normal drawn on the line AB at point
NRT → Normal drawn on the line AC at point

Here, it has been assumed that the material of the prism is denser than the surrounding medium.

Question 2. What is the condition for an emergent ray to be directed away from the base of a prism? Show that in this condition net deviation is negative.

Answer: If the optical density of the material of the prism is less than the optical density of the surrounding medium, then the emergent ray will be directed away from the base.

According to the given condition, a ray diagram showing refraction in the prism is drawn.

According to image, r₁ >  i₁ and i₂ < r₂ Thus, ( r₁ + r₂) > (i₁+ i₂)
Therefore deviation, δ = (i₁ + i₂) ~ ( r₁+ r₂) <0
i.e., net deviation is negative.

Question 3. Show that during refraction of light along the principal section of a prism, angle of deviation, δ = i₁ + i₂ – A, where i₁ is the angle of incidence, i₂ is the angle of emergence and A is the refracting angle of the prism.

Answer: ABC is the principal section of a prism. PQRS is the path of light in refraction through.

For refraction on the plane AB, the angle of incidence, ∠PQM =  i₁ and the angle of refraction, ∠RQT = r₁

∴ the angle of deviation in refraction on the plane AB is δ₁ = ∠EUR = i₁ – r₁

For the refraction on the plane AC, the angle of incidence, ∠QRT = r₂ and the angle of refraction, ∠NRS = i₂

∴ the angle of deviation in refraction on the plane AC, δ₂ = ∠FRS = i₂ – r₂

If SR is extended backwards, it intersects PE at the point U. Therefore, due to refraction of a ray of light at two surfaces AB and AC, angle of deviation,
δ = ∠EUR [It is the external angle of ΔUQR ] = ∠UQR + ∠QRU = ∠UQR + ∠FRS [ ∵ ∠QRU= vertically opposite ∠FRS] =  δ₁ + δ₂
= (i₁ –  r₁) + (i₂  – r₂) = i₁ +  i₂ -( r₁ + r₂) …..(1)

We get from ΔQTR, r₁ + r₂ + ∠QTR = 180° •••(2)
Again, for quadrilateral AQTR,
∠AQT + ∠ART = 90° + 90° = 180°
∴ ∠QAR + ∠QTR = 180° ……..(3)

By comparing equations (2) and (3), we get ∠QAR =  r₁ + r₂ or, A =r₁ + r₂
∴ we get from equation (1), the angle of deviation δ = i₁ + i₂ -A

Question 4. No deviation occurs in refraction through a rectangular glass slab—-prove it.

Answer: A light ray AB is incident obliquely on the side PQ of a rectangular slab PQRS.

 

 

In case of refraction of light at point B, angle of incidence, i₁= ∠ABN₁ and angle of refraction, r₁ = ∠N₂ BC

Ray BC passes through this glass and falls obliquely at the point C on the surface of separation of glass and air and is again refracted in air. Here CD is the emergent ray. In case of refraction of light at point C, angle of incidence, r₂ = ∠BCN₃ and angle of refraction, i₂= ∠DCN₄ 

Here, N₁N₂ is parallel to N₃N₄ and BC is a transversal.
∴ ∠N₂BC = ∠BCN₃ [Alternate angles] or, r₁ = r₂

Again, it can be shown from Snell’s law, If r₁ = r₂, then  i₁ =i₂  

Therefore total deviation, δ = ( i₁ – r₁) + (r₂—i₂) = (i₂  – i₂ ) + (r₂ – r₂ ) =0

i.e., no deviation occurs in refraction through a rectangular glass slab.

Question 5. What is the refraction of light?

Answer: The passage of light ray from one medium to another medium through the surface of separation of the two media is called refraction of light.

Alternate definition: During passage of light from a transparent homogeneous medium to another transparent homogeneous medium, light suffers a change in speed at the surface of separation of the two media. This phenomenon of change in light of light at the surface of separation of the two media is called refraction of light.

Common Mistakes in Light Calculations

Question 6. Write down the laws of refraction of light.

Answer:

The laws of refraction of light are:

(1)The incident ray, the refracted ray and the normal to the surface of separation of the two media at the point of incidence lie on the same plane.
(2)The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. This constant depends on the nature of the associated media and colour of incident light.

Question 7. What do you mean by the optical density of a medium? Is it related to the physical density of the medium?

Answer:

Optical density of a medium

1. Optical density is the property of a medium on which the speed of light depends. More the optical density of a medium, slower the ray moves through it.

2. The optical density of a medium is not the same as its physical density. The physical density (ρ) of any substance is denoted by the ratio of its mass (M) and volume (V), i.e.., \(\rho=\frac{m}{V}\)

But the optical density of a medium is indicated by its refractive index (µ) which is calculated as µ = c/v (c = speed of light in vacuum, v = speed of light in the said medium).

Question 8. What do you mean by refractive index of a medium?

Answer:

Refractive index of a medium

The ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) at the time of refraction, in case of oblique incidence is called the refractive index of the second medium (µ₂) with respect to the first medium (µ₁), i.e., \(\frac{\sin i}{\sin r}={ }_1 \mu_2\)

Question 9. When is the formula \(\frac{\sin i}{\sin r}=\mu_2\) not applicable?

Answer; If a ray of light is incident normally at the surface of separation of two media, then the formula \(\frac{\sin i}{\sin r}=\mu_2\) is not applicable. In this case, there is no change in the direction of the ray of light in the second medium but speed of light change in the second medium.

Question 10. Refraction takes place from medium a to medium b. Explain which one is optically denser or rarer under the following conditions:
(1)Positive deviation occurs in medium b.
(2)Negative deviation occurs in medium b.

Answer: If angle of incidence = i and angle of refraction = r, then deviation, δ = (i – r).

(1)As deviation in medium b is positive, So, i – r > 0 or, i > r i.e., r < i.

In this case, the value of angle of refraction in medium b is less than the value of angle of incidence in medium a. Therefore, medium b is denser than medium a.

(2)As deviation in medium b is negative, So, δ < 0 or, i – r < 0  or, i<r, i.e.,r>i

In this case, the value of angle of refraction in medium b is greater than the value of angle of incidence in medium a. Therefore, medium b is rarer than medium a.

Question 11. Show the refraction of light
(1)From a rarer medium to medium and from a denser medium to a rarer medium
(2)From a denser medium to a rarer medium.

Answer:
(1)When a ray of light enters a denser medium from a rarer medium, the refracted ray bends towards the normal.
(2)When a ray of light enters a rarer medium from a denser medium, the refracted ray bends away from the normal.

 

In Image,
AO → incident ray
OB → refracted ray
O → point of incidence
MON→ normal drawn on the point of incidence at the surface of separation of two media
i → angle of incidence
r → angle of refraction

In case of (1), r < i
In case of (2), r > i

Question 12. Determine the angle of deviation Sn each of the following cases:
(1)Refraction from rarer to denser medium
(2)Refraction from denser to rarer medium

Answer: The refraction of light from a rarer medium to a denser medium and from a denser medium to a rarer medium respectively. Here AOC is the undeviated path of light in absence of any second medium.
So, ∠BOC = the angle of deviation (δ).

(1)In refraction of light from a rarer medium to a denser medium, r < i.
We get δ = ∠BOC = ∠CON – ∠BON = ∠AOM – ∠BON [∵ ∠AOM – vertically opposite ∠CON] or, δ = i-r

(2)In refraction of light from a denser medium to a rarer medium, r > i.
δ = ∠BOC = ∠BON -∠CON = ∠BON-∠AOM [ ∵∠AOM = vertically opposite ∠CON]
∴ δ = r-i

Question 13. Explain which medium is rarer and which one is denser between the two media a and & in the phenomenon of refraction Under the following conditions:

1. Angle of incidence is 30° in medium a, angle of refraction is 40q in medium b .
2. Angle of incidence is 50° in medium a , angle of refraction is 40° in medium b.
3. Incident ray in medium o is inclined at an angle of 30° with the surface of separation and refracted ray in medium b is inclined at an angle of 40° with the surface of separation.
4. Incident ray in medium a is inclined at an angle of 50° with the surface of separation and refracted ray in medium b is inclined at an angle of 40° with the surface of separation.
5. Incident ray falls perpendicularly at the surface of separation in medium a.

Answer:
(1)In this case, angle of refraction (40°) > angle of incidence (30°).
Thus, the refracted ray bends away from the normal in medium b. Therefore, medium b is rarer compared to medium a.

(2)In the case, angle of refraction (40°) < angle of incidence (50°)
Thus the refracted ray bends towards the normal inmedium b. Therefore, medium b is denser than the medium a.

(3)In this case, angle of incidence (i) = 90°-30° = 60° and angle of refraction (r) = 90° – 40° – 50° So; r < /’, i.e., the refracted ray bends towards the normal in medium b.

Therefore, medium b is a denser medium than medium a.

 

(4)In this case, angle of incidence(i) = 90°-50° = 40° and angle of refraction (r) = 90° – 40° = 50° r>i, i.e., the refracted ray bends away from the normal in medium b.

Therefore, medium b is rarer in comparison to medium a.

(5)In this case, angle of incidence (i) = angle of refraction (r) = 0

As a result, from given data it is not possible to know which medium is optically rarer or denser.

Question 14. If a ray of light incidents on a parallel glass slab at an angle i, the angle between the reflected ray and the refracted ray is 90°. Show that refractive index of glass, µ = tani.

Answer:

Given

If a ray of light incidents on a parallel glass slab at an angle i, the angle between the reflected ray and the refracted ray is 90°.

Suppose, angle of refraction = r
AB is the incident ray, BC is the reflected ray and BD is the refracted ray.
∴∠CBD = 90°

NN₁ is the normal at point B on the surface of separation between the two media.

∴ ∠NBC+ ∠CBD + ∠N₁BD = 180° or, i + 90 + r = 180° or, r = 90° – i

In case of refraction of light ray at point B, \(\frac{\sin i}{\sin r}=\mu \quad \text { or, } \frac{\sin i}{\sin \left(90^{\circ}-i\right)}=\mu \quad \text { or }, \frac{\sin i}{\cos i}=\mu\)

Question 15. Why is a glass rod immersed in glycerine invisible?

Answer: We see an object due to the irregular reflection of light from its surface. The lesser the amount of this reflection, lesser will be the visibility of the object to us. Again, the lesser the difference between the refractive indices of two media greater will be the amount of refracted rays.

This means diffuse reflection from the surface of separation is reduced. Here, the refractive indices of glycerine and glass are almost equal. So, when a glass rod is immersed in glycerine, practically no reflection of light occurs at the surface of the separation. Maximum amount of light is refracted from glycerine to glass. So, the glass rod is not visible from the outside.

 

 

Question 16. Explain the following parts for a prism:

1. Refracting surface,
2. Edge,
3. Base,
4. Principal section,
5. Refracting angle or angle of the prism.

Answer:
(1)Refracting surface: The two rectangular surfaces DEHG and DFIG are called refracting surfaces of this prism.

 

 

(2) Edge: The edge of the prism is the straight line along which the two refracting surfaces of the prism meet. Here DG is the edge of the prism.
(3)Base: The rectangular plane opposite to the edge is called the base. Here, EFIH is the base.
(4)Principal section: It is an imaginary triangular plane surface right-angled to the edge of the prism. This is called the principal section of the prism. Here, ABC is the principal section of the prism.

(5)Refracting angle: The angle between the two refracting surfaces of the prism is called the refracting angle or simply the angle of the prism. Here, ∠BAC is the refracting angle of the prism.

Question 17. If the value of angle of incidence of a ray on one surface of a prism (i₁) increases, what is the change in the angle of emergence (i₂) at the other surface?

Answre: If the value of angle of incidence of a ray on one surface of a prism ( i₁) increases, the value of angle of refraction (r₁) on the same surface also increases according to Snell’s law.

Now, as the refracting angle of the prism A = r₁ + r₂ is constant, the value of r₂ decreases with the increase in the value of r₁.

Again, for the refraction at the second surface, as the value of r₂ decreases, the value of i₂ also decreases according to Snell’s law.
So, if the value of i₁ increases, the value of i₂ decreases.

 

Topic B Refraction of Light Very Short Answer Type Questions Choose The Correct Answer

 

Question 1. The following remains unchanged during refraction of light

1. Speed of light
2. Wavelength
3. Frequency
4. None of these

Answer: 3. Frequency

Question 2. If the angle of incidence for a prism is increased, angle of deviation

1. Increases
2. Decreases
3. Increases at first, then decreases
4. Decrease at first, then increases

Answer: 3. Increases at first, then decreases

Question 3. For which colour of light among red, blue, green and yellow does a medium possess highest refractive index?

1. Red
2. Blue
3. Green
4. Yellow

Answer: 2. Blue

Question 4. For which colour of light among red, blue, green and yellow does a medium possess lowest refractive index?

1. Red
2. Blue
3. Green
4. Yellow

Answer: 1. Red

Question 5. For which value of the angle of incidence in refraction of light, the equation \(\frac{\sin i}{\sin r}={ }_1 \mu_2\) is not valid?

1. 90°
2. 60°
3. 45°
4. Zero

Answer: 4. Zero

Question 6. Refraction of light occurs from denser to a rarer medium. Angle of incidence is 30° and angle of refraction is 45°. What is the angle of deviation?

1. 10°
2. 15°
3. 20°
4. 30°

Answer: 2. 15°

Question 7. A ray of light is incident from air to the upper surface of a liquid at an angle 45° and is refracted at an angle of 30°. What is the refractive index of that liquid?

1. 2
2. √2
3. 1 + √2
4. √2 -1

Answer: 3. 1 + √2

Question 8. The angle of deviation of a prism with refracting angle A is δ, when the angle of incidence is i₁ and the angle of emergence is i₂. What is the angle of deviation, for the angle of incidence i₂ in the same prism?

1. δ/2
2. δ/3
3. δ
4. 2δ

Answer: 3. δ

Question 9. What will be the angle of incidence at the second refracting surface of a. prism of refracting angle 60° for angle of refraction 25° at the first refracting surface?

1. 35°
2. 40°
3. 25°
4. 15°

Answer: 1. 35°

Question 10. A ray of light incident on an equilateral glass prism, if incident angle on first face is 30° and emergent angle is 50° then angle of deviation is

1. 10°
2. 20°
3. 30°
4. 40°

Answer: 2. 20°

Question 11. The absolute refractive index of water is |. What is the refractive index of air with respect to that of water?

1. 3/4
2. 4/3
3. 3/5
4. 1

Answer: 1. 3/4

Question 12. Angle of deviation in case of refraction with a normal incidence is

1. 90°
2. 60°
3. 30°
4. Zero

Answer: 4. Zero

Question 13. When light wave goes from air into water, the quality that remains unchanged is its

1. Speed
2. Amplitude
3. Wavelength
4. Frequency

Answer: 4. Frequency

Question 14. When light travels from a denser to a rarer medium, its speed

1. Remains same
2. Decreases
3. Increases
4. Can’t be said

Answer: 3. Increases

Question 15. If the frequency of light in vacuum be v, then the frequency of light in a medium of refractive index n will be

1. µν
2. 0
3. µ/ν
4. ν

Answer: 4. ν

Question 16. Which one of the following alternative is not true for a prism placed in a position of minimum deviation

1. i₁ = i₂
2. r₁ = r₂
3. i₁ = r₁
4. None of these

Answer: 3. i₁ = r₁

Question 17. During refraction of light from air to a transparent liquid, angle of incidence is 60° and angle of refraction is 45°. What is the angle of deviation?

1. 10°
2. 30°
3. 15°
4. 20°

Answer: 3. 15°

 

Topic B Refraction of Light Answer In Brief

 

Question 1. A ray of light, while entering from one medium into another, bends towards the normal. What conclusion can be drawn about the optical density of the media?
Answer: One can conclude that the first medium is the rarer medium and the second medium is the denser medium.

Question 2. A ray of light, while entering from one medium into another medium, bends away from the normal. What conclusion about the optical density of the media can be drawn from this?
Answer: One can conclude that the first medium is the denser medium and the second medium is the rarer medium.

Question 3. What is the angle of deviation when the angle of incidence and the angle of refraction are 60° and 45° respectively?
Answer: Angle of deviation = 60° – 45° = 15

Question 4. The refractive indices of two particular media for red light and blue light are µ_r and µ_b respectively. Which is greater in magnitude?
Answer: Here, µ_b > µ_r.

Question 5. What is the value of angle of refraction if the angle of incidence is zero?
Answer: If the angle of incidence is zero, then the angle of refraction is also zero.

Question 6. What is the angle of deviation between the incident ray and the emergent ray when a ray of light fails on a rectangular glass slab?
Answer: The angle of deviation is zero in this case.

Question 7. Do the incident ray and the emergent ray remain in the same straight line when an incident ray falls obliquely on a rectangular glass slab?
Answer: No, the emergent ray gets displaced laterally.

Question 8. In which direction does a ray of Sight incident on a prism parallel to its base bend after it emerges—towards the base of the prism or towards the refracting angle?
Answer: A ray of light incident on a prism parallel to its base, bends towards its base after it emerges.

Question 9. A ray of light is incident on a prism parallel to its base where the refractive index of the surrounding medium is greater than the refractive index of the material of the prism. In which direction does the emergent ray bend—towards the base or towards the refracting angle?
Answer: In this case, the emergent ray bends towards the refracting angle.

 

Topic B Refraction of Light Fill In The Blanks

Question 1. If one looks from above at a pencil immersed obliquely in water, it looks bent due to ____ of light.
Answer: Refraction

Question 2. The speed of light ______ when it travels from water to glass medium.
Answer: Decreases

Question 3. The angle of incidence is ______ than the angle of refraction when light travels from a rarer to a denser medium.
Answer: More

Question 4. The angle of incidence is ____ than the angle of refraction when light travels from a denser to a rarer medium.
Answer: Less

Question 5. Absolute refractive index of a medium (except air or vacuum) is always greater than ______
Answer: One

Question 6. When light travels from a rarer to a denser medium, its speed _____
Answer: Decreases

Question 7. The position of prism with respect to the incident ray at which the incident ray suffers minimum deviation is called the position of ______
Answer: Minimum Deviation

 

Topic B Refraction of Light State Whether True Or False

 

Question 1. When light is normally incident on a glass slab, only lateral displacement of light takes place.
Answer: False

Question 2. Incident ray and refracted ray may not always lie on the same plane.
Answer: False

Question 3. Snell’s law is not applicable for normal incidence of light.
Answer: True

Question 4. Glass is a rarer optical medium than diamond.
Answer: True

Question 5. The frequency of light decreases when it enters into glass from water.
Answer: False

Question 6. If angle of incident be 45° and the angle of refraction be 30°, then the refractive index be √2.
Answer: True

Question 7. For refraction of light through a rectangular glass slab angular deviation of light beam is zero.
Answer: True

Question 8. For an isosceles prism, when the deviation is minimum the path of the ray through the prismbecomes parallel to the base of the prism.
Answer: True

 

Topic B Refraction of Light Numerical Examples

 

1. For refraction of light, if angle of incidence = i, angle of refraction =r, then

(1)For refraction of light from rarer medium to denser medium, angle of deviation, δ = i – r
(2)For refraction of light from denser medium to rarer medium, angle of deviation,  δ  = r-i
(3)Refractive index of the second (2) medium with respect to the first (1) medium, \({ }_1 \mu_2=\frac{\sin i}{\sin r}\)

2. If refractive index of a medium with respect to air be µ. Then refractive index of air with respect to the medium = 1/µ
3. If absolute refractive index of 1st medium be µ₁ absolute refractive index of 2nd medium be µ₂ then, refractive index of the 2nd medium with respective to the 1st medium be \({ }_1 \mu_2=\frac{\mu_2}{\mu_1}\)

4. For refraction of light through a prism, if angle of the prism —A, angle of incidence at the first refracting surface = i₁, angle of refraction at the first refracting surface = r₁ and at the second refracting surface, angle of incidence =r₁, angle of refraction = i₂, then
(1)A =  r₁ + r₂
(2) Angle of deviation 8 = i₁ + i₂ – A
(3)For minimum deviation, i₁ = i₂ – i (say) and r₁ = r₂ and µ_m = 2i- A or, \(i=\frac{\delta_m+A}{2}\)

Question 1. An Incident ray of Tight travelling through air falls on the upper surface of a liquid at an angle of 45° – It is deviated by 15° after refraction in that liquid. What is the refractive index of the liquid?

Answer:

Given

An Incident ray of Tight travelling through air falls on the upper surface of a liquid at an angle of 45° – It is deviated by 15° after refraction in that liquid.

Angle of refraction, r = i-δ or, r = 45° – 15° or, r = 30°

∴ refractive index of the liquid, \(\mu=\frac{\sin i}{\sin r}\)

or, \(\mu=\frac{\sin 45^{\circ}}{\sin 30^{\circ}}\)  or, \(\mu=\frac{\frac{1}{\sqrt{2}}}{\frac{1}{2}}=\sqrt{2}=1.414\)

Question 2. What is the value of the angle of deviation if the angle of incidence on one surface of an equilateral prism is 30° and the angle of refraction on the other surface is 50°?

Answer: Refracting angle of an equilateral prism, A = 60°
According to this question, angle of incidence on one surface,  i₁= 30°

Angle of refraction at the other surface, i₂ = 50° a
∴ angle of deviation, δ = i₁ + i₂ — A = 30° + 50° – 60° = 20°

WBBSE Class 10 Revision Notes on Light

Question 3. The angle of deviation of a ray of light, due to refraction in an equilateral prism is 40°. If the ray of light travels in a path parallel to the base of the prism, what Is the value of the angle of incidence on the first surface?

Answer:

Given

The angle of deviation of a ray of light, due to refraction in an equilateral prism is 40°. If the ray of light travels in a path parallel to the base of the prism,

The path of ray, in the equilateral prism ABC.

∵ QR || BC
∴ ∠AQR =∠ABC = 60°
∴ ∠RQO = 90° – 60° = 30°

Hence angle of refraction on the first surface,  r₁ = 30°
∵A = r₁ + r₂
∴ 60° = 30° +r₂  or, r₂ = 30°
∵ r₁=  r₂
∴  i₁ = i₂ = i (say)
∴ δ = i₁ + i₂ – A

∴\(i=\frac{\delta+A}{2}=\frac{40^{\circ}+60^{\circ}}{2}=50^{\circ}\)

Therefore, the value of angle of incidence = 50°.

 

 

Question 4. What is the value of angle of emergence (or angle of refraction) at the second surface of a prism with a refracting angle of 30° for the normal incidence of the ray on the first surface of the prism? [Given, the refractive index of the material of the prism, µ = 1.5 and sin 48.59° = 1.5/2]

Answer: The refracting angle of the prism, A = 30°
The angle of incidence on the first surface, i₁ = 0 [ ∵ the ray is incident normally]
Therefore, angle of refraction, r₁ = 0
Again, since A = r₁+ r₂
∴ 30° = 0 + r₂ or, r₂ = 30°

If i₂ be the angle of emergence (angle of refraction) in the second surface, then according to Snell’s law, \(\frac{\sin r_2}{\sin i_2}=\frac{1}{\mu}\)

[∵ here refraction is from prism (refractive index = µ) to air (refractive index = 1)]

or, sini₂ = µ sinr₂ = 1.5sin30° = 1.5/2
∵ sin 48.59° = 1.5/2(given)
∴ i₂ = 48.59°

Word Problems on Reflection and Refraction

Question 5. The refracting angle of a prism is 30°. If a ray of light falls at an angle of 60° on a refracting surface, what is the angle of emergence? [Given, angle of deviation = 30° ]

Answer:

Given

The refracting angle of a prism is 30°. If a ray of light falls at an angle of 60° on a refracting surface,

Refracting angle of the prism, A = 30°
Angle of incidence, i₁ = 60°
Angle of deviation, δ = 30°
Suppose, the angle of emergence = i₂

∴ δ = i₁ + i₂ -A  or, i₂ = δ – i₁  + A = 30°- 60° + 30° = 0
∴ The angle of emergence = 0

Question 6. The refractive index of glass is 3/2 and that of water is 4/3. What is the refractive index of glass with respect to water?

Answer:

Given

The refractive index of glass is 3/2 and that of water is 4/3.

Refractive index of glass \(\mu_g=\frac{3}{2}\)

Refractive index of water \(\mu_w=\frac{4}{3}\)

∴ Refractive index of glass with respect to water \(\mu_g=\frac{\mu_g}{\mu_w}=\frac{\frac{3}{2}}{\frac{4}{3}}=\frac{9}{8}\)

Question 7. The refracting angle of a prism is 60°. If a ray is incident on any refracting surface of the prism, angle of refraction for that surface is 35°. What is the incident angle on the other refracting surface?

Answer:

Given

The refracting angle of a prism is 60°. If a ray is incident on any refracting surface of the prism, angle of refraction for that surface is 35°.

Refracting angle of the prism, A = 60°, angle of refraction on the first refracting surface r₁ = 35°

If r₂ is the angle of incidence on the other refracting surface,
A =  r₁+ r₂   or, r₂ = A-r₂  or, r₂ = 60°-35° = 25°

Light Topic C Lenses Synopsis

1. Lens is a portion of a transparent refracting medium bounded by one or two spherical surfaces or one spherical surface and one plane surface.

 

 

 

2. A lens whose both the refracting surfaces are spherical in shape that it is thick at the middle and thin at the edge is called a convex lens. It converges the parallel beam of light incident on the lens after refraction. It is also known as converging lens.

3. A lens whose both the refracting surfaces are spherical in shape such that it is thin at the middle and thick at the edge is called a concave lens.
It diverges the parallel beam of light incident on the lens after refraction. It is also known as diverging lens.

4. Converging/diverging action of convex/ concave lens: Converging or diverging action of lens can be shown by considering action of lens to be made up of large number of prism.

 

 

 

 

 

 

5. Technical terms related to lenses:
(1)The centre of the sphere whose part is the lens surface, is called the centre of curvature of that surface of the lens.
(2)Since some lens has two spherical surfaces, so there are two centres of curvature of a lens.
(3)The radius of the sphere whose part is the lens surface, is called the radius of curvature of that surface of lens.
(4)The line joining the centres of curvature of the two surfaces of the lens is called principal axis of the lens.
(5)Optical centre is a point on the principal axis of the lens such that a ray of light passing through this point emerges parallel to its direction of incidence.
(6)A light ray can pass through a lens from either direction, therefore, a lens has two principal foci which are situated at equal distance (when the medium on both sides of the lens is same) from the optical centre, one on either side of the lens. These are known as the first principal focus and the second principal focus.

First principal focus is a point on the principal axis such that rays coming from it (convex lens) or appear to meet at it (concave lens) become parallel to the principal axis after refraction.

 

 

 

 

 

Second principal focus is a point on the principal axis at which the rays incident on the lens parallel to the principal axis after refracting converges (in convex lens) or appear to diverge (in concave lens).

 

 

6. The distance between the focal point ( F₁ or  F₂) and the optical centre of the lens is called the focal length of the lens.
7. Usually, when we say focus, we mean the second focal point. Hence the focal length of a lens implies the second focal length of the lens.

8. Rules for obtaining images formed by lenses (concave or convex):
(1) An incident ray diverging through the first principal focus or appearing to converge at first principal focus emerges parallel to the principal axis after refraction.
(2)An incident ray passing through the optical centre passes through the lens without deviation after refraction.
(3)An incident ray parallel to the principal axis after refraction converges (in a convex lens) or appears to diverge (in a concave lens) through the second focus.

9. A Conlens can form a real as well as a virtual image but concave lens always form a virtual, erect and diminished image.

Cartesian sign convention:
(1) Light is incident on the lens from the left-hand side. a direction opposite to the incident light
(2)All distances are measured from the and normal to the principal axis are taken as the optical centre of the lens which is taken as the origin and the principal axis as X-axis.
(3)The distances taken in the direction of the incident light from the optical centre are taken as negative.
(4)Heights and distances measured upward And normal to the principal axis are taken as positive while downward distances are taken
as negative.

Position and nature of image formed by a convex lens at different positions of object:

10. Magnification (m) produced by a lens is defined as the ratio of the size of the image to that of the object. The size of the object h₀ is always taken to be positive, but image size h_i is taken as positive for an erect image and negative for an Inverted image, \((m)=\frac{h_i}{h_0}=\frac{v}{u}\)

11. The human eye is one of the most Important biological instrument which consists of— cornia, iris, pupil, ciliary muscles, eye tens, retina and optic nerve.
12. Defects of the human eye: Two main defects of human eye are—
(1)Myopia or short sight
(2)Hypermetropia or long sight.

(1)Myopia is that defect in vision in which a person can see nearby objects clearly but can not see the distant object clearly.
(2)Hypermetropia is that defect in vision in which a person can see the distant objects clearly but cannot see the nearby objects clearly.

12. Least distance of distinct vision: The nearest point at which an object can be seen distinctly is called near point of the eye. The shortest distance at which an object can be seen distinctly is called the least distance of distinct vision. It is 25 cm for normal human eye.

13. Far Point is the farthest point upto which an object can be distinctly seen without accommodation.

For a normal eye the far point is at infinity. The distance between the near point and the far point is called range of vision.

14. Persistence of vision: The retina of the eye continues to bear the effect of light after the stimulus has been taken away. This phenomenon is called the persistence of vision. Persistence of vision is about 1/10 s.

 

Light Topic C Lenses Short And Long Answer Type Questions

 

Question 1. Write down the definition of the second principal focus, focal length and focal plane of a convex lens with Images.

Answer:
Second principal focus: A beam of light incident parallel to the principal axis of a convex lens converges at a point on the principal axis after refraction through the lens. This point is called the second principal focus F₂ of the convex lens.

Focal length: The distance between the optical centre and the principal focus of a lens is called its focal length. In The Image OF₂ is the second principal focus of a convex lens.

Focal plane: An imaginary plane perpendicular to the principal axis of the lens and passing through its principal focus, is called the focal plane of the Jens.

 

 

PQRS is the focal plane of the convex lens.

Question 2. Define the optical centre of a lens with a figure. What is the deviation of the ray passing through the optical centre of a lens.

Answer:

The optical centre of a lens with a figure:

1. If an emergent ray of light from a lens becomes parallel to the incident ray on it, then the point of intersection of the ray with the principal axis is called the optical centre of the lens.

AB is the incident ray on a convex lens and CD is the emergent ray. Further, AB is parallel to CD. In this case, the ray BC intersects the principal axis at a point O. O is the optical centre of the lens.

2. The ray passing through the optical centre of a lens makes no deviation.

Question 3. What is a thin lens? Define its optical cenmter with a figure.

Answer:

Thin lens:

1. A lens is called a thin lens when its thickness is negligible compared to the radii of curvature of the two spherical surfaces.
2. Optical centre of a thin lens is a special point on its principal axis so that any ray of light passing through it in passing through the lens suffers neither later displacement nor a net deviation.

 

 

 

 

Question 4. An image of a candle is formed on the wall with the help of a lens. The size of the image is equal to the size of the candle. Here the distance between the candle and the wall is 80 cm. What is the nature of this lens? What is its focal length?

Answer:

Given:

An image of a candle is formed on the wall with the help of a lens. The size of the image is equal to the size of the candle. Here the distance between the candle and the wall is 80 cm.

1. In this case, a real image of the same size as that of the object has been formed with the help of the lens. Only a convex lens can form a real image. So, the lens is definitely a convex lens.

2. If an object is placed in front of the lens at a distance of twice the focal length (2ƒ), a real image of the same size as the object is formed.
2ƒ + 2ƒ= 80 or, ƒ= 20
i.e., the focal length of the lens is 20 cm.

Question 5. If a lens is kept at a distance of 12 cm in front of a wall, an inverted, diminished image of a distant tree is formed on the wall. What is the nature of the lens? What Is the value of the focal length of the lens?

Answer:

1. The lens is convex in nature because only a convex lens can form the real image of an object placed in front of it.
2. A beam of light rays from a distant tree may be regarded as a beam of parallel rays. A beam of parallel rays form a real image at the focal plane after refraction through the lens. As the distance of the wall on which the image is formed is 12 cm from the lens, the focal length of the lens is 12 cm (approx).

Question 6. What type of lens Is used in a simple camera? What is the characteristic of an image taken by a camera on its photographic plate?

Answer:

1. A convex lens is used in a simple camera.
2. A real image is formed on the photographic plate in a camera. So, it is inverted with respect to the object. The size of the image compared to the size of the object depends on the distance of the object from the lens.

Question 7. Show that, linear magnification of a lens can be expressed by the ratio of image distance and object distance.

Answer: O₁O₂ is an extended object kept perpendicularly on the principal axis of a convex lens and a concave lens respectively and I₁I₂ is its image.

Let us suppose that object distance, OO₂=u and image distance, OI₂ = ν.

We get from the images that in ΔOO₁O₂ and ΔOI₁I₂, ∠O₁OO₂ = ∠I₁OI₂(opposite angle),
and ∠O₁O = ∠I₁I₂O(both are right angles)
and ∠O₂O₁O = ∠OI₁I₂(alternate angle)

∴ ΔO₂O₁O and Δ OI₁I₂
\(\frac{O_1 O_2}{I_1 I_2}=\frac{O O_2}{O I_2} \text { or, } \frac{I_1 I_2}{O_1 O_2}=\frac{O I_2}{O O_2}\)
and hence magnification, \(m=\frac{v}{u}\)

Question 8. What do you mean by focusing in a camera? If the position of the lens of a camera remains fixed> is it possible to take a clear photograph of an object at an arty distance? Explain.

Answer:
1. Focusing in a camera is the formation of an image on photographic film with the help of the lens of the camera.
2. The image is clearest when there is correct focusing. To form a clear image of an object placed at different distances from the lens, it is essential that the objects are focused separately. There is an arrangement to move the lens of the camera forward and backward for proper focusing. Without this facility of movement of the lens, we can take clear photograph of an object placed only at one particular distance.

Question 9. Draw the diagram of a human eye and label the different parts. Show how an image is formed in the eye with the help of a ray diagram.

Answer:

 

Question 10. Write down the definition of the terms ‘near point’, ‘far point’ and ‘range of vision’ of an eye and what are the values of these for a normal eye?

Answer:
Near point: The nearest point that we can see clearly by our eyes without any adjustment is the near point The distance of this near point from a normal eye is approximately 25 cm.
Far point: The farthest point that we can see clearly by our eyes without any adjustment is the far point. The far point for a normal eye is situated at infinity.
Range of vision: The distance between the near point and the far point is called the range of vision of the eye. For a normal eye, this distance ranges from 25 cm to infinity.

Question 11. What is hypermetropia? Explain the defect of long-sightedness with proper diagram.

Answer:

Hypermetropia:

1. If one can see distant objects but not the nearby objects clearly, then this defect is called long-sightedness or hypermetropia.
2. If the near point of the eye of an individual is displaced, the defect of long-sightedness is observed.

Let N be the near point of a normal eye. Here, the image of the point N is formed at a point behind the retina instead of getting focused at the retina. The eye forms the image of the nearest point N’ at retina by maximum adjustment. In this case, N’ is the near point of an eye with the defect of long-sightedness.

 

 

Question 12. What is the remedy for the defect of short-sightedness? Explain with a diagram.

Answer:

The remedy for the defect of short-sightednes:

1. If a pair of spectacles with concave lenses of suitable focal lengths are used, the defect of short-sightedness can be remedied.
2. Let the near point of an individual with the defect of long-sightedness be N’ and the near point for the normal eye be N. The focal ength of the convex lens is such that after refraction through the lens, the virtual image of the point N is formed at N’. As a result, the point N appears to be at the point N’.

 

 

Question 13. What is myopia? Explain the defect of short-sightedness with proper diagram.

Answer:

Myopia:

1. If one can see the nearby objects but not the distant objects very clearly, then this defect is called short-sightedness or myopia.
2. A beam of parallel rays coming from a distant object, after refraction by the lens of the eye is focused at a point A in front of the retina instead of getting focussed at the retina. In this case, the far point is displaced to some point in front of the eyes instead of being located at infinity.

 

 

Question 14. What Is the remedy for the defect of long-sightedness? Explain with proper diagram.

Answer:

Remedy for the defect of long-sightedness

1. If a pair of spectacles with convex lenses of suitable focal lengths are used, the defect of long sightedness can be remedied.
2. The focal length of the concave lens is such that the incident parallel rays coming from any distant object, after refraction through the concave lens, appear to diverge from a point F and the image is formed at the retina.

 

 

Question 15. Write down the differences between an eye and a camera.

Answer:

The differences between an eye and a camera

Eye	Camera
1. The focal length of the lens of the eye can be changed.	1. The focal length of the objective of a camera is fixed.
2. The distance between the lens of the eye and the screen or retina is fixed.	2. The distance between the objective of the camera and the screen or film can be changed.
3. Innumerable images are formed on the retina but none of them is permanent.	3. One image is formed in one film of the camera and a permanent photograph is obtained from that image.

 

Question 16. What is a lens? How many types of lenses are there? Mention them.

Answer:

Lens

1. A lens is a transparent refracting medium bounded by two spherical surfaces or one spherical surface and another plane surface.
2. There are two types of lenses. One is called a convex lens and the other is called the concave lens.

Question 17. What are convex lenses and concave lenses?

Answer:
Convex Lens: A lens whose middle portion is thick and whose two edges become thin gradually, is called a convex lens.
Concave Lens: A lens whose middle portion is thin and whose two edges become gradually thick is called a concave lens.

Question 18. Why is a convex lens called a converging lens and a concave lens called diverging lens?

Answer: If a beam of parallel rays of light is incident on a convex lens, they are converted into a beam of converging rays after refraction through the lens. That is why a convex lens is called a converging lens.

Again, if a beam of a parallel rays of light is incident on a concave lens, they are converted into a beam of diverging rays of light after refraction through the lens. That is why a concave lens is called a diverging lens.

 

 

Question 19. Define the first principal focus of a convex lens with a proper diagram.

Answer:

The first principal focus of a convex lens with a proper diagram:

The first principal focus of a convex lens is a point located on the principal axis of the lens, such that the divergent rays coming from it, become parallel to the principal axis of the lens after refraction through the lens. F₁ the first principal focus of a convex lens.

 

 

Question 20. Define the first principal focus of a concave lens with a proper diagram.

Answer:

The first principal focus of a concave lens with a proper diagram

The first principal focus of a concave lens is a point on the principal axis such that an incident beam of light rays directed towards it, emerges parallel to the principal axis after refraction through the lens.

 

 

_F1 is the first principal focus of a concave lens.

Question 21. Define the second principal focus of a concave lens with diagram, identify the focal distance and the focal plane in the diagram.

Answer: A beam of light incident parallel to the principal axis of a concave lens appears to diverge from a point on the principal axis after refraction by the lens. This point is called the second principal focus F₂ of the lens.

 

 

OF₂ is the focal length of the lens and PQRS is the focal plane.

Question 22. Define the following with figures for a lens:
(1)Centre of curvature
(2)Radius of curvature
(3)Principal axis

Answer:
(1)The centre of the sphere, of which any of its two surfaces of a lens is a part, is called the centre of curvature of that particular surface of the lens.

The centres of curvature of the right and the left spherical surfaces of the convex lens are C₁ and C₂ respectively.
The centres of curvature of the left and the right spherical surfaces of the concave lens are C₁ and C₂ respectively.

(2)The radius of the sphere of which the spherical surface of a lens is a part is called the radius of curvature of that sphere.

 

 

C₁L₁  and C₂L₁ are the radii of curvature of the right and the left spherical surfaces of a convex lens. C₁P₁ and C₂P₂ are the radii of curvature of the left and the right spherical surfaces of the concave lens.

(3)If the two surfaces of a lens are spherical, the line joining the two centres of curvature is called the principal axis of the lens. In the image C₁ and C₂  is the principal axis of both the convex and concave lens respectively.

Question 23. Explain the following with figures:
(1)Secondary focus of a convex lens
(2) Secondary focus of a concave lens

Answer:
(1)A parallel beam of incident light inclined to the principal axis of a convex lens meets at a point on the focal plane after refraction through the lens. This point is called the secondary focus. F’ is the secondary focus of the convex lens.

 

 

(2)A parallel beam of incident light inclined to the principal axis of a concave lens appears to diverge from a point located on the focal plane of the lens after refraction through the lens. This point is called the secondary focus. F’ is the secondary focus of the concave lens.

 

 

(3)A parallel beam of incident light inclined to the principal axis of a concave lens appears to diverge from a point located on the focal plane of the lens after refraction through the lens. This point is called the secondary focus. F’ is the secondary focus of the concave lens.

Question 24. Explain the converging action of a convex lens.

Answer;

The converging action of a convex lens:

Let us suppose that a beam of light parallel to the principal axis is incident on a convex lens, To get an idea about the nature of refraction of this parallel beam of rays, the lens is considered to be the sum of a very large number of small sliced (truncated) prisms.

The bases of these prisms above and below the principal axis are oriented towards the principal axis. When a ray of light is incident on any small sliced prism parallel to the principal axis, it bends towards the base of the prism due to refraction.

As a result, the parallel rays of the beam meet at a particular point on the principal axis after refraction. In this way, a convex lens transforms a beam of parallel rays to a beam of converging rays. That is the reason why a convex lens is also known as a converging lens.

 

 

Question 25. Explain the diverging action of a concave lens.

Answer:

The diverging action of a concave lens:

Let us suppose that a beam of light parallel to the principal axis is incident on a concave lens. To get an idea about the nature of refraction of this parallel beam of rays, the lens is considered to be an aggregate of a very large number of small sliced (truncated) prisms.

The bases of these prisms above and below the principal axis are oriented away from the principal axis of the concave lens. When an incident ray falls on any small sliced prism in a direction parallel to the principal axis, it bends towards the base of the prism due to refraction.

As a result, the parallel rays of the beam appear to diverge from a particular point on the principal axis after refraction. In this way a concave lens transforms a beam of parallel rays to a beam of diverging rays. That is the reason why a concave lens is also known as a diverging lens.

 

 

Question 26. What is a linear magnification of a lens?

Answer:

A linear magnification of a lens:

The ratio of the height of the image of an object to the height of the object kept perpendicular to the principal axis of the lens, is known as the linear magnification of the lens.

If the heights of the object and the image are and h₂, then linear magnification, \(m=\frac{h_2}{h_1}\)

Question 27. Nature of the image is given below. Draw a ray diagram of the formation of image of an extended object by a convex lens. Mention the position of the object, the position of the image and the practical application in each of the following cases:
(1)Very small real image compared to the size of the object
(2)A Real image of a smaller size than the object
(3)A real image of the same size as that of the object
(4)A real image of size larger than that of the object
(5)Virtual Image

Answer:

Nature of the image	Position of the object	Position of the image	Practical application
Very small real image compared to the size of the object	An infinite distance from the lens	On the focal plane of the lens on the side of the lens other than that on which the object is placed	The objective of a telescope is made by utilising this property of a convex lens
2. A real image of smaller size than the object	At a distance more than twice the focal length of the lens	On the side of the lens other than that on which the object is placed and at a distance between ƒ and 2ƒ	This property of a convex lens is used in a camera

 

 

 

 

Nature of the image	Position of the object	Position of the image	Practical application
3. A real image of the same size as that of the object	At a distance of 2ƒ from the lens	At a distance of 2ƒ from the lens on the side of the lens other than that on which the object is placed	This property of the convex lens is utilised in a terrestrial telescope to convert an inverted image into an erect one
4. A real image of size larger than that of the object	The object is positioned between ƒ and 2ƒ from the lens	At a distance of more than 2ƒ from the lens on the side of the lens other than that on which the object is placed	The objective of a microscope utilises this property of a convex lens

 

 

 

Nature of the image	Position of the object	Position of the image	Practical application
5. Virtual image	The object is placed between the lens and the focus	The image is produced on the side as that of the object but behind the object	In this case, the virtual image is always larger in size than the object. A magnifying glass works according to this principle.

 

 

 

Question 28. A candle kept very close to a convex lens of focal length ƒ Is slowly removed from the lens. Different positions of the candle are given below. Mention the position of the image, its size and nature in each of the given cases. Position of the candle:
1. At a distance less than ƒ from the lens
2. At focus
3. More than ƒ but less than 2ƒ distance from the lens
4. At a distance 2ƒ from the tens
5. More than 2ƒ distance from the lens
6. At a very large distance from the lens

Answer:

Position of the candle	Position of the image	Size and nature of the image
1. At a distance less than ƒ	At a distance of more than ƒ on the same side of the lens as the object	Magnified, erect, virtual
2. At the focus	At infinity, on the other side of the lens on which the object is placed.	Highly magnified, inverted, real
3. Distance is more than ƒ but less than 2ƒ	At a distance of more than 2ƒ on the other side of the lens on which the object is placed.	Magnified, inverted, real
4. At a distance of 2ƒ	At a distance of 2ƒ on the other side of the lens on which the object is placed	Same size, inverted, real
5. At a distance more than 2ƒ	At a distance more than ƒ but less than 2ƒ on the other side of the lens on which the object is placed	Diminished, inverted, real
6. At a very large distance	At the second principal focal plane	Highly diminished, inverted, real

 

Visual Representation of Light Phenomena

Question 29. Why the convex lens is called as a magnifying glass? State uses of magnifying glass in practice.

Answer:
1. If an object (or writings) is placed within the focus of a convex lens and is looked through it, the object looks magnified. Hence the convex lens is called as a magnifying glass.

 

 

2. A magnifying glass is used to get an enlarged view of a small object. For example, it is used to read the small writing on a bottle of syrup or on a medicine packet. Further, it is used to help a watch mechanic to see the very small instruments of a watch or to note the fine readings in a laboratory.

Question 30. Draw a ray diagram showing the image formation of an object by a concave lens.

Answer: L₁L₂  is a thin concave lens with a small aperture. O₁O₂ is an extended object placed perpendicularly on the principal axis of the concave lens.

 

Here the image is virtual. It is erect and diminished in size.

Question 31. How do you recognise whether a lens is a convex lens Or a concave lens?

Answer: It is known that if an object is placed in front of a convex lens at a distance less than its focal length, an erect, magnified and virtual image of the object is formed. On the other hand, a concave lens always forms an erect, diminished and real image.

A pencil is kept in front of a lens and it can be seen through the lens from the other side. If a magnified and erect image is seen, the lens is convex and if the image is diminished and erect, the lens is concave.

Question 32. The focal length of a convex lens is 15 cm. Suppose you want to read the small writings on a bottle of syrup. Where will you keep the lens?

Answer: We know that if a convex lens is kept in front of an object within a distance of its focal length, an erect, magnified and virtual image of the object in formed. So, in order to read the small writings on a bottle of syrup, the bottle must be kept within a distance of 15 cm from the lens. The writings look magnified from the other side of the lens.

Question 33. On which factors does the focal length of a lens depend?

Answer: The focal length of a lens depends on the following factors:
1. Refractive index of the material of the lens and the surrounding medium.
2. Radii of curvature of the two spherical surfaces of the lens.

Question 34. What is a conjugate pair of foci of a lens?

Answer:

Conjugate pair of foci of a lens

By keeping the position of the lens fixed, if two points are located on the principal axis such that for a point object placed at one, an image is formed at the other, then these two points are called conjugate pair of foci.

Question 35. what Is the change in the image of an object If one half of a convex lens is covered with a black paper?

Answer: As one half of the lens is still exposed, light refracts through that portion and a full image of the object is formed since there is no refraction of light through the other half, the brightness of the image reduces.

Question 36. Why is a human eye compared to a camera?

Answer: When an object is placed in front of a camera, a picture (real image) is produced on the film of the camera. Similarly when an object is placed in front of the eye, a real image of that object is produced on the screen of the eye (retina). This is why a human eye is compared to a camera.

Question 37. What do you mean by accommodation and adaptation of an eye?

Answer:

Accommodation and adaptation of an eye

Accommodation is the process by which the human eye changes optical power to focus on an object as its distance varies (from 25 cm to infinity). The pupil of the eye controls the amount of light entering the eye by its automatic contraction and expansion. This ability of the eye is called adaptation.

Question 38. What is the role of different colours of light to form the colour of a transparent body? Write down with examples.

Answer:

The role of different colours of light to form the colour of a transparent body

1. When light of a particular colour emerges from a transparent body (like air, water, or glass), the body appears to be of the same colour as the colour of the light.
2. For example, only red light can pass through a red glass. If light of any other colour passes through it, it is absorbed. That is why the piece of glass looks red. Again, if a blue glass is placed below a red glass and if it seen from the top, the blue glass looks black due to the reason that if the blue colour goes through a red glass, red glass absorbs it.

Question 39. How does a red species of china rose appear at night in a room lighted with blue light?

Answer: If a red species of china rose flower is seen at night in a room lighted with blue light, it appears black. Since red species of china rose absorbs all colours except red, if the blue light of the room falls on this flower, it absorbs that and no light is reflected by it.

Question 40. What do you mean by the statement that the colour of an object Is white or black?

Answer:

The colour of an object Is white or black

Black and white are not fundamental colours. If an object absorbs all colours then it looks black and if it reflects all the seven colours of the sunlight, it looks white. For example, if sunlight falls on an umbrella, it absorbs all the incident light and no light is reflected. So a black umbrella appears black. Again, if the rays of the sun fall on a white cloth, the cloth reflects all the incident colours and so white cloth appears white.

Question 41. If a white flower is viewed through a blue glass, how does it look?

Answer: If a white flowers is viewed through a blue glass, it appears blue. The seven colors reflected from the white flower enter the blue glass and the glass absorbs all the colours except the blue one. Only blue light reaches the observer and so the flower appears blue.

Question 42. How does the sun look if it is viewed through red glass?

Answer: If rays of the sun fall on a red glass, it absorbs all the light except the red one and then only red rays reach the eyes of a viewer. So the sun appears red.

Question 43. How does it look if a red and a blue glass are kept together and then viewed from the front?

Answer: If the sun’s rays fall on a red glass, it absorbs all the colours except the red one. So only red light is incident on the blue glass. But blue glass again absorbs red light. As a result, no light reaches the observer.

Question 44. Moon has no atmosphere. What can you determine logically about the colour of the sky of the moon?

Answer: There is no atmosphere in the moon, so there is no scattering of sunlight in moon. As no light comes to the surface of the moon from its sky, the sky of the moon appears black.

Light Topic C Lenses Very Short Answer Type Questions Choose The Correct Answers

Question 1. The number of principal focus of a lens is

1. One
2. Two
3. Three
4. Four

Answer: 2. Two

Question 2. A convex lens forms a magnified real image of an object

1. Placed at a distance less than ƒ
2. Placed at a distance more than but less than 2ƒ
3. Placed at a distance of 2ƒ
4. Placed at a distance of more than 2ƒ

Answer: 2. Placed at a distance more than but less than 2ƒ

Question 3. The values of object distance and image distance in a convex lens are 10 cm and 20 cm respectively. What is linear magnification?

1. 1/2
2. 2
3. 1/4
4. 4

Answer: 2. 2

Question 4. The distance of the near point of a normal eye is about

1. 20 cm
2. 25 cm
3. 30 cm
4. 50 cm

Answer: 2. 25 cm

Question 5. If we place a red glass on a blue glass and observe from the top, what colour does the blue glass convey?

1. Blue
2. Red
3. Yellow
4. Black

Answer: 4. Black

Question 6. For a concavo-convex lens

1. The radius of curvature of convex surface is less than the radius of curvature of concave surface
2. The radius of curvature of convex surface is equal to the radius of curvature of concave surface
3. The radius of curvature of concave surface is less than the radius of curvature of convex surface
4. One surface is convex and the other surface is plane

Answer: 1. The radius of curvature of convex surface is less than the radius of curvature of concave surface

Question 7. For a convexo-concave Sens

1. The radius of curvature of convex surface is less than the radius of curvature of concave surface
2. The radius of curvature of convex surface is equal to the radius of curvature of concave surface
3. The radius of curvature of convex surface is more than the radius of curvature of concave surface
4. One surface is concave and the other surface is plane

Answer: 3. The radius of curvature of convex surface is more than the radius of curvature of concave surface

Question 8. The optical centre of a plano-convex lens is located

1. At the point of intersection of the principal axis and the plane surface
2. The principal axis is outside the lens but nearer to the plane surface
3. At the point of intersection of principal axis and the convex surface
4. On the principal axis and is outside the lens but nearer to the convex surface

Answer: 3. At the point of intersection of principal axis and the convex surface

Question 9. A convex lens forms a magnified virtual image of an object when the object distance is

1. Equal to ƒ
2. Equal to 2ƒ
3. Less than ƒ
4. More than 2ƒ

Answer: 3. Less than ƒ

Question 10. A convex lens forms a diminished real image of an object when

1. ƒ< u < 2ƒ
2. u < ƒ
3. u = ƒ
4. u>2ƒ

Answer: 4. u>2ƒ

Question 11. A convex lens forms a real image of the same size of an object when the object distance is

1. Less than ƒ
2. Equal to 2ƒ
3. More than 2ƒ
4. More than / but less than 2ƒ

Answer: 2. Equal to 2ƒ

Question 12. When a convex lens forms a virtual image of magnification m then

1. m > 1
2. m = l
3. m < 1
4. m ≤ 1

Answer: 1. m > 1

Question 13. A concave lens forms

1. Always a magnified virtual image
2. Always a diminished virtual image
3. Always a virtual image of the same size as the object
4. Amagnified real image

Answer: 2. Always a diminished virtual image

Question 14. The focal length of a lens depends

1. Only on the colour of the incident light
2. Only on the refractive index of the materials of the lens with respect to the surrounding medium
3. Only on the radius of curvature of the two spherical, surfaces
4. On all the above three

Answer: 4. On all the above three

Question 15. If an object is kept at a distance of more than / but less than 2/ in front of a convex lens, then image distance is

1. Less than ƒ on the other side
2. More than 2ƒ on the other side
3. More than ƒ on the same side as the object
4. 2ƒ on the other side

Answer: 2. More than 2ƒ on the other side

Question 16. If an object is kept at a distance of more than 2ƒ  in front of a convex lens, then image distance is

1. More than 2ƒ on the other side
2. More than ƒ but less than 2ƒ on the other side
3. More than ƒ on the same side as the object
4. More than 2ƒ on the other side

Answer: 2. More than ƒ but less than 2ƒ on the other side

Question 17. If an object is kept at a distance of less than f in front of a convex lens, then the image distance is

1. Always less than the object distance
2. Less or more than the object distance
3. Always more than the object distance
4. Less or equal to the object distance

Answer: 3. Always more than the object distance

Question 18. If an object is kept in front of a concave lens, then the image distance is

1. Always more than the the object distance
2. Always less than the object distance
3. More or equal to the object distance
4. More or less than the object distance

Answer: 2. Always less than the object distance

Question 19. To minimise the reflection of light, the inner walls of a pin-hole camera should be coloured in

1. White
2. Red
3. Yellow
4. Black

Answer: 4. Black

Question 20. A person with a defect of long-sightedness should use

1. Convex lens
2. Concave lens
3. Cylindrical lens
4. Toric lens

Answer: 1. Convex lens

Question 21. A person with a defect of short-sightedness should use

1. Convex lens
2. Concave lens
3. Cylindrical lens
4. Toric lens

Answer: 2. Concave lens

Question 22. A convex lens

1. Converges light rays
2. Diverges light rays
3. Form real images always
4. Form virtual images always

Answer: 1. Converges light rays

Question 23. Near and far points of human eye are

1. 25 cm and infinity
2. 50 cm and infinity
3. Infinity and 0 cm
4. Ocrn and 25 cm

Answer: 1. 25 cm and infinity

Question 24. For a normal eye, the least distance of distinct vision is

1. 0.25 m
2. 0.5 m
3. 25 m
4. Infinity

Answer: 1. 0.25 m

Question 25. Image formed on the retina is

1. Real and inverted
2. Virtual and erect
3. Real and erect
4. Virtual and inverted

Answer: 1. Real and inverted

Question 26. The impact of an image on the retina remains for

1. 0.1s
2. 1 s
3. 0.5 s
4. 10 s

Answer: 1. 0.1s

Question 27. The transparent watery liquid acts as the refracting medium of light and occupies the space between the cornea and the eye lens is

1. Aqueous humour
2. Vitreous humour
3. Tear
4. Glycerin

Answer: 1. Aqueous humour

 

Light Topic C Lenses Answer In Brief

 

Question 1. Which lens is known as a converging lens?
Answer: A convex lens is known as a converging lens.

Question 2. Which lens is known as a diverging lens?
Answer: A concave lens is known as a diverging lens.

Question 3. Can the optical centre of a lens be outside the lens?
Answer: Yes, the optical centre of a lens can be outside the lens. For example, concavo-convex lens, and convexo-concave lens.

Question 4. How many principal focal points of a lens are there?
Answer: There are two principal focal points of a lens.

Question 5. How many secondary focal points of a lens are there?
Answer: There are infinite number of secondary focal points of a lens.

Question 6. What is the nature of the path of the refracted rays when the rays incident on a lens are directed towards the optical centre?
Answer: If the incident rays are directed towards the optical centre, the rays get refracted along the same path.

Question 7. Which lens always forms a virtual image?
Answer: A concave or diverging lens always forms a virtual image.

Question 8. At a particular position of an object in front of a lens, an image of the same size as the object is formed. What is the nature of the Sens?
Answer: The lens is convex in nature.

Question 9. In which lens, a virtual image is formed which is smaller than the object?
Answer: Such an image is formed by a concave lens.

Question 10. Which type of lens is used as a magnifying glass?
Answer: A convex lens is used as a magnifying glass.

Question 11. If a finger is viewed from the opposite side of a lens, the finger looks smaller. What is the nature of the lens?
Answer: The lens is concave in nature.

Question 12. If a finger is viewed from the opposite side of a lens, the finger looks larger. What is the nature of the lens?
Answer: The lens is convex in nature.

Question 13. Where should a convex lens of focal length 10 cm be placed in order to read the small letters of a book?
Answer: The lens has to be placed at a distance of less than 10 cm from the book.

Question 14. Does a camera form a real or a virtual image of an object?
Answer: A real image of the object is formed in a camera.

Question 15. What is the main function of the retina?
Answer: The main function of the retina is to form an image of the object.

Question 16. Which portion of the retina does not contain any photosensitive cell?
Answer: The blind spot of the retina does not contain any photosensitive cell.

Question 17. Speed of light in two media A and B are 2 x 10⁸ m/s and 2.25 x 10⁸ m/s respectively. Which one is the rarer medium?
Answer: Speed of light is greater in medium B than that in medium A. So, B is the rarer medium.

Question 18. Speed of light in two media X and Y are 1.5 x 10⁸ m/s and 1.9 x 10⁸ m/s respectively. Which one is the denser medium?
Answer: Speed of light in medium X is smaller than that in Y. So, X is the denser medium.

Question 19. Which wavelength of light does produce a sensation of vision in our eyes?
Answer: Light of wavelength 4000A to 8000A produces a sensation of vision in our eyes.

 

Light Topic C Lenses Fill In The Blanks

 

Question 1. The pupil is present at the centre of _____
Answer: Iris

Question 2. The near point is situated at a longer distance than normal distance for a man with ________ sightedness.
Answer: Long

Question 3. The far point is situated at a shorter distance than normal distance for a man with ________ sightedness.
Answer: Short

Question 4. If a convex lens is immersed in water, its focal length ______
Answer: Increases

Question 5. A _____ lens always forms a virtual image of an object.
Answer: Concave

Question 6. A ______ lens is used in photography.
Answer: Convex

Question 7. An object is placed at a distance 2ƒ from a convex lens of focal length ƒ. The size of image formed is that of the object.
Answer: Equal

Question 8. A magnifying glass is a _______ lens.
Answer: Convex

 

Light Topic C Lenses State Whether True Or False

 

Question 1. When a ray of light passes through the optical centre of a thin lens, there is no deviation or lateral displacement of the light ray.
Answer: True

Question 2. A concave lens kept in the air medium acts as a converging lens.
Answer: False

Question 3. A person suffering from near-sightedness cannot see the nearby objects clearly.
Answer: False

Question 4. The colour of an opaque object is that it absorbs.
Answer: False

Question 5. If an object does not reflect any colour at all, it appears to be black in colour.
Answer: True

Question 6. The colour of the rays of light that emerge from a transparent object appears to be the colour of the object.
Answer: True

Question 7. In case of a concavo-convex lens optical centre lies out side the lens.
Answer: True

Question 8. A thin lens is one of which the thickness at the principal axis is small compared with the radii of curvature of the two surfaces.
Answer: True

Question 9. Air bubble into water acts like a concave lens.
Answer: True

Question 10. A magnifying glass is concave lens.
Answer: False

Question 11. Image formed by the convex lens may be magnified and virtual.
Answer: True

 

Light Topic C Lenses Numerical Examples

 

1. For refraction through a lens, if object distance = u, image distance = v, height of the object = h₁ height of the image = h₂

The linear magnification, \(m=\frac{\text { image height }}{\text { object height }}=\frac{h_2}{h_1}=\frac{v}{u}\)

Question 1. The focal length of a convex lens is 10cm. Where should an object be placed in front of the lens so that the image formed is

1. Magnified and real
2. Diminished and real
3. Of the same size and real
4. Magnified and virtual

Answer: Focal length of the lens, ƒ = 10 cm
∴ 2ƒ= 2 x 10 = 20 cm
Let us suppose that object distance = u

1. To get a magnified and real image, ƒ<u<2ƒ or, 10 cm < u < 20 cm is the condition.
2. To get a diminished and real image, u > 2ƒ or, u > 20 cm is the condition.
3. To get a real image of the same size, u = 2ƒ or, u = 20 cm is the condition,
4. To get a magnified and virtual image, u < ƒor, u < 10 cm is the condition.

Wb Class 10 Physical Science Question and Answers

Question 2. Focal length of a convex lens is 10 cm. An object of height 2 cm is placed at a distance 20 cm from the lens, What is nature and magnification of the image?

Answer: Focal length of the convex lens is ƒ= 10 cm,
object distance u = 20 cm
Here, u = 20 cm.= 2 x 10 cm = 2ƒ
Therefore, image distance v = 2ƒ= u = 20 cm
As, u> ƒ thus image will be real.

Height of the object  h₁ = 2 cm and height of the image = h₂ say ,
Now, magnification of the image,\(\frac{h_2}{h_1}=\frac{v}{u}\)
or, \(\frac{h_2}{2}=\frac{20}{20}\)
∴ h₂ = 2cm
∴ Height of the image is 2 cm and the image is real, inverted.

Question 3. Height of an object is 5 cm. When the object is placed in front of a convex lens at a distance 2 cm from the lens, an image of 10 cm height is obtained. What is image distance and the magnification of the object?

Answer: Object height = h₁ = 5cm, image height = h₂ = 10 cm , object distance = u = 2 cm , image distance = v (say)

∴ Linear magnification of the image \(m=\frac{h_2}{h_1}=\frac{10}{5}=2\)

Again, \(\frac{h_2}{h_1}=\frac{v}{u}\) or, \(\frac{v}{2}=\frac{10}{5} \quad \text { or, } v=\frac{10 \times 2}{5}=4\)

.’.So the image distance is 4 cm.

Question 4. When an object is placed in front of a convex lens at a distance 15 cm from the lens, image of the object is formed in opposite side of the lens at a distance 30 cm from the lens. Find the magnification and height of the image if object height is 1.5 cm.

Answer: Object distance u = 15 cm ,
Image distance v = 30 cm
∴ Magnification of the image = \(\frac{v}{u}=\frac{30}{15}=2\) and height of the image, (h₂) = m x object height = 1.5 x 2 = 3 cm

 

Light Topic D Dispersion Of Light And Light Wave Synopsis

 

1. When a beam of polychromatic light enters into a refracting medium obliquely, it splits into its component colours. This phenomenon is called dispersion of Sight.
2. Spectrum is the sequence of the colours obtained after the dispersion of white light.
3. Rainbow is a natural example of the dispersion of light.

4. The causes of dispersion: In vacuum or air all the different colours of light rays travel at the same speed. But through any other medium, they travel with different speed and gets refracted through different angles. Thus cause of dispersion is the change in speed of light.

5. Colour of different bodies: Colour of a body depends on 0 the colour of incident light 0the portion of light absorbed by them.

6. Colour of an opaque body: A opaque body appears in that colour which is reflected by the body.

7. Colour of transparent body: The colour of a transparent object determined by the colour it can transmit when illuminated by white light.

Wb Class 10 Physical Science Question and Answers

8. Primary colours: Red, blue and green colours are called primary colours.

9. Complementary colours: Any two special colours which on mixing together give the sensation of white are known as complementary colours.
Example: blue + yellow = green + magenta = red + peacock blue = white

10. Light rays of wavelengths 4000A to 8000A creates a sensation of vision of our eyes.
11. Light is an electromagnetic transverse wave. This wave does not affected by the electric and magnetic field.

∴ Speed of light in vacuum c = νλ, where ν and λ are the frequency and wavelength of light wave respectively.

12. Dust particles or air molecules have smaller dimensions than the wavelength of incident visible light. These particles absorb the incident light and then emit the same wave. This phenomenon is called the scattering of light.
13. Scientist Rayleigh proved that the intensity of scattered light (l) is inversely proportional to the fourth power of the wavelength ( λ ) of light, i.e„ \(I \propto \frac{1}{\lambda^4}\)

 

Light Topic D Dispersion Of Light And Light Wave Short And Long Answer Type Questions

Question 1. What is a dispersion of light? What is a spectrum? What is the sequence of colours in the spectrum of white light?

Answer:
1. A polychromatic light breaks into different colours while passing through a refracting medium. This phenomenon is called dispersion of light.
2. The band that is obtained due to the dispersion of polychromatic light in different colours is known as the spectrum.
3. The spectrum that is produced due to the dispersion of white light through a prism has red colour at the top and violet colour at the bottom. The colours are arranged from bottom to top in the following order: violet, indigo, blue, green, yellow, orange and red.

Question 2. With the help of an experiment, show that a prism does not create colour but disperses a polychromatic light.

Answer: A prism does not create colours, it simply separates the different colours present in white light. An experiment can be performed to demonstrate this. A beam of white light is incident on prism P₁ through a small slit S. The rays emerging from the prism form a spectrum VR on the screen.

There is another small slit S₁ in screen C₁. By moving the screen up and down, any ray of the spectrum (say, yellow-coloured ray) is sent through S₁ and is made to fall on another prism P₂. The emergent rays from P₂ fall on another screen C₂. It can be seen that this light is not divided into different colours, i.e., no spectrum is formed. The same thing happens for other colours. This proves that the prism does not create any colour.

 

 

Question 3. Speeds of light in water and glass are 2.25 x 10⁸ m/s and 2 X 10⁸ m/s respectively. Which medium is optically denser?

Wb Class 10 Physical Science Question and Answers

Answer: Refractive index of water, \(\mu_w=\frac{c}{v_w}\) and that of glass, \(\mu_g=\frac{c}{v_g}\)

∴ \(\frac{\mu_w}{\mu_g}=\frac{\frac{c}{v_w}}{\frac{c}{v_g}}=\frac{v_g}{v_w}=\frac{2 \times 10^8 \mathrm{~m} / \mathrm{s}}{2.25 \times 10^8 \mathrm{~m} / \mathrm{s}}=\frac{2}{2.25}<1\)

 

i.e.., \(\mu_w<\mu_g \quad \text { or, } \mu_g>\mu_w\)
As refractive index of glass is greater than that of water, glass is the optically denser medium.

Question 4. Why is there no dispersion of light when white light falls on a rectangular slab of glass?

Answer: Let assume that a beam of white light is incident on a face of a rectangular glass slab. This beam of light is composed of innumerable rays. As the refractive index of glass is different for seven constituent colours of white light, dispersion occurs for each ray at the surface of separation.

 

 

Now as the slab is a rectangular one when these spectra emerge from the other face of the slab, they overlap with each other to form white light. Only lights at the two ends (red and violet) are refracted, whose amount is negligible as compared to the amount of refracted white light. So, there is no dispersion is found in a rectangular glass slab.

Question 5. What is a visible spectrum?

Answer: Electromagnetic waves which have wavelengths between 4000A and 8000 produce a sensation of vision in our eyes. The waves within this range constitute a visible spectrum.

Question 6. What type of wave is light? What is the relationship between the speed of light wave and the frequency of light?

Answer:
1. Light is an electromagnetic transverse wave.
2. The wavelength of a light wave is λ. If the frequency of light in any medium is ν, then speed of light in that medium, v = ν λ.

Question 7. Write down the uses of ultraviolet rays and also its harmful effects.

Answer: Uses of ultraviolet rays:
1. As a sterilising agent.
2. For detecting purity of gems, ghee, eggs etc 0 In manufacturing vitamin D.
3. In the treatment of skin infections like psoriasis.

Harmful effect: Excess exposure to ultraviolet ray may cause skin cancer.

Question 8. Write down the uses of X-rays and also their harmful effects.

Answer: Uses of X-rays:
1. To detect fractures in bones or the inner structure of teeth.
2. In the treatment of cancer and also in research for causes of different diseases.
3. To study the structure of crystals.
4. In forensic investigation.

Harmful effect: It may damage a normal cell if the cell is exposed to it for a prolonged period of time.

Question 9. Write down the uses of γ-ray and also its harmful effect.

Answer: Uses of  γ-ray:
1. In radiotherapy to destroy cancerous cells.
2. It is used in the industry to check if there is any gap in casting or moulding.

Harmful effect: It may damage a living cell to a great extent.

Question 10. What is the relation of the intensity of scattered light with its wavelength?

Answer: Scattering is not the same for light of all wavelengths. Light with a high wavelength has less scattering and light with a low wavelength has more scattering. Scientist Rayleigh proved that the intensity of scattered light is inversely proportional to the fourth power of the wavelength (λ) of light, i.e. \(I \propto \frac{1}{\lambda^4}/latex]

Question 11. what Is the role of different colours of light to form the colour of an opaque body? Write down with examples.

Answer: If an opaque body is kept in sunlight, it appears to be of the same colour which it reflects. If the body reflects more than one colour, it appears to have that colour which is produced by mixing those reflected colours.

For example, it can be said that when sunlight falls on a red species of china rose, it absorbs all colours except red. As it reflects only red colour, red species of chinarose looks red. Again, a leaf looks green because when sunlight falls on a leaf, it reflects only green light.

Wb Class 10 Physical Science Question and Answers

Question 12. Why does the sky look blue during day?

Answer: Rays of the sun travel a long distance through the atmosphere before coming to the earth’s surface. Small dust particles and molecules of gas present in the atmosphere cause the scattering of this sunlight.

As the wavelength of red light is greater than other lights in the visible spectrum, scattering of red light is less. But for blue or violet lights, scattering is more as the wavelength of these lights is less. Our eyes are more sensitive to blue light than violet, so the sky appears blue to us.

Question 13. Why is red fight used as a danger signal?

Answer: Red light has the longest wavelength among the lights of the visible spectrum. As a result, red light has the lowest amount of scattering. That is why red light is used as a danger signal.

Question 14. Why does the sun appear red during sunset and sunrise?

Answer: The sun remains at the horizon during sunrise or sunset, so the rays of the sun have to travel a long distance through the atmosphere. Among the seven constituent colours of sunlight, red light has the longest wavelength and so the scattering for it is the least.

The other lights with smaller wavelengths have greater scattering and as a result, do not reach the earth’s surface. Only red light reaches the earth. So, the sun appears red during sunset and sunrise.

Question 15. Why yellow light is used as a fog light in a vehicle?

Answer: Fog scatters the visible light in different directions, so we cannot see distant objects. As the wavelength of yellow light is comparatively greater, scattering is less and it is easy to see through the fog. But a scattering of red light is lesser but eyes are more sensitive to yellow light than red light. That is why yellow light is used as a fog light in a vehicle.

 

Light Topic D Dispersion Of Light And Light Wave Very Short Answer Type Questions Choose The Correct Answer

 

Question 1. In dispersion of white light with a prism, the colour appears on the top of the screen is

1. Violet
2. Red
3. Yellow
4. Green

Answer: 2. Red

Question 2. In the dispersion of white light with a prism, the colour that appears at the bottom of the screen is

1. Violet
2. Blue
3. Red
4. Orange

Answer: 1. Violet

Question 3. If the wavelength of light is A and its frequency is v, then the speed of light in a vacuum is

1. ν/λ
2. [latex]\frac{\lambda}{v}\)
3. νλ
4. \(\sqrt{v \lambda}\)

Answer: 3. νλ

Question 4. If a light wave of wavelength 6000 A enters water (\(\mu_w=\frac{4}{3}\) its wavelength will becomes

1. 4000 A
2. 4500 A
3. 6000 A
4. 8000 A

Answer: 2. 4500 A

Question 5. If a wave of frequency 1.6 x 10¹⁴ Hz Enters water \(\mu_w=\frac{4}{3}\), its wavelength becomes

1. 1.2 × 10¹⁴ Hz
2. 1.6 × 10¹⁴ Hz
3. 2.3 × 10¹⁴ Hz
4. 1.5 × 10¹⁴ Hz

Answer: 2. 1.6 x 10¹⁴ Hz

Wb Class 10 Physical Science Question and Answers

Question 6. Among the following electromagnetic waves, which one has the longest wavelength?

1. Ultraviolet radiation
2. Visible light wave
3. Microwaves
4. X-rays

Answer: 3. Microwaves

Question 7. Among the following electromagnetic waves, which one has the highest energy?

1. Radiowaves
2. Microwaves
3. Infrared waves
4. Visible light wave

Answer: 4. Visible light wave

Question 8. The relation between the intensity (I) of scattered light and its wavelength (λ) is

1. \(I \propto \lambda\)
2. \(I \propto \frac{1}{\lambda^2}\)
3. \(I \propto \lambda^4\)
4. \(I \propto \frac{1}{\lambda^4}\)

Answer: 4. \(I \propto \frac{1}{\lambda^4}\)

Question 9. Which colour does scatter least?

1. Violet
2. Blue
3. Yellow
4. Green

Answer: 3. Yellow

Question 10. Which colour does scatter most?

1. Green
2. Yellow
3. Orange
4. Red

Answer: 1. Green

Question 11. Among the following colours of light whose velocity is maximum while passing through a glass slab?

1. Red
2. Blue
3. Yellow
4. Violet

Answer: 1. Red

Question 12. What will, be the colour of the sky as seen from the earth, if there is no atmosphere

1. Black
2. Blue
3. Red
4. Green

Wbbse Class 10 Physical Science Question and Answers

Answer: 1. Black

Question 13. Sun set or sun rise, the sun looks redder than at mid-day because of

1. The scattering effect of light
2. The effect of refraction
3. The effect of diffraction
4. The sun is hottest at this time

Answer: 1. The scattering effect of light

Question 14. The colour of the sky is blue due to

1. Bending of light beam
2. Dispersion of light
3. Scattering of light
4. Reflection of light

Answer: 3. Scattering of light

 

Light Topic D Dispersion Of Light And Light Wave Answer In Brief

 

Question 1. In case of dispersion of white light a prism, which colour has the maximum angle of emergence?
Answer: Red light has the maximum angle of emergence.

Question 2. In case of dispersion of white light a prism, which colour has the minimum angle of emergence?
Answer: Violet light has the minimum angle of emergence.

Question 3. In an electromagnetic spectrum, which one has the minimum wavelength (or highest frequency)?
Answer: γ-ray in an electromagnetic spectrum has the minimum wavelength (or highest frequency).

Question 4. What is the speed of light in vacuum?
Answer: The speed of light in vacuum is 3 x 10⁸ m/s.

Question 5. In an electromagnetic spectrum, which one has the maximum wavelength (or lowest frequency)?
Answer: Radio wave in an electromagnetic spectrum has the maximum wavelength (or lowest frequency).

Question 6. Which property of light explains the blue colour of the sky during day?
Answre: Blue colour of sky duringday can be explained by the property of scattering of light.

Question 7. Which ray among visible spectrum scatters the most?
Answer: Violet ray scatters the most.

Question 8. Which colour is used as a danger signal?
Answer: Red colour is used as a danger signal.

Question 9. Which ray among visible spectrum scatters the least?
Answer: Red ray scatters the least.

 

Light Topic D Dispersion Of Light And Light Wave Fill In The Blanks

 

Question 1. The refractive index of any medium is _______ for red light than for yellow light.
Answer: Less

Question 2. γ-ray is _______ more powerful than X-ray.
Answer: More

Question 3. The wavelength of microwave is _______ than that of radio waves.
Answer: Less

Question 4. ______ ray is used to determine the structure of a crystalline material.
Answer: X

Wb Class 10 Physical Science Question and Answers

Question 5. In radiotherapy, ______ -ray is used to destroy the cancer cells of a patient.
Answer: γ

Question 6. If the speed of light in a medium is 1.5 x 10⁸ m/s, refractive index is _______
Answer: 2

Question 7. _____ of scattered light is inversely proportional to the λ⁴.
Answer: Intensity

Question 8. The phenomenon of splitting up of polychromatic light into different colours is called _______ of light.
Answer: Dispersion

Question 9. A Rainbow is the natural phenomenon ______ of light.
Answer: Dispersion

Question 10. Red, green and blue these three colours are called _____ colours.
Answer: Primary

Question 11. _____ light is always used as a danger signal.
Answer: Red

Question 12. Glass is a _____ medium.
Answer: Dispersive

Question 13. Any two colours which on mixing together gives the sensation of white are known as ______ colours.
Answer: Complementary

 

Light Topic D Dispersion Of Light And Light Wave State Whether True Or False

 

Question 1. White light is a monochromatic light.
Answer: False

Question 2. A prism does not create colour.
Answer: True

Question 3. Glass is a dispersion medium whereas vacuum is not.
Answer: True

Question 4. Material medium is necessary for propagation of electromagnetic wave.
Answer: False

Question 5. The colour of an opaque object is determined by the colour it absorbed.
Answer: False

Wb Class 10 Physical Science Question and Answers

Question 6. The colour of a transparent object is determined by the colour it can transmit.
Answer: True

Question 7. Red glass absorbs all other colours except red.
Answer: True

Question 8. If a body reflects no light, it looks black.
Answer: True

Question 9. A red rose appears blue when viewed through green glass.
Answer: False

Question 10. The speed of γ-rays and that of X-rays are different in vacuum.
Answer: False

Question 11. The wavelength of microwaves is greater than that of radio wave.
Answer: False

Question 12. The intensity of scattered wave \(l \propto \frac{1}{\lambda^2}\) where λ is the wavelength of light.
Answer: False

 

Light Topic D Dispersion Of Light And Light Wave Numerical Examples

 

1. Speed of light in vacuum c = 3 x 10⁸ m/s
2. If speed of light in a medium be v, then absolute refractive index of the medium, \(\mu=\frac{c}{v}\)

3. If v₁ and v₂ be the speed of light in medium-1 and medium-2 respectively then, \({ }_1 \mu_2=\frac{\mu_2}{\mu_1}=\frac{v_1}{v_2}\)

4. If frequency and wavelength of light in vacuum be ν and λ respectively, then speed of light c = ν x λ

5. If wavelength of light in vacuum be λ and the same in a medium be λ’ then, \(\mu=\frac{\lambda}{\lambda^{\prime}}\)

Question 1. What Is the frequency of X-ray of wavelength 1.2A?

Answer:
Frequency of X-ray = v (say)
∴ Speed of X-ray, c = ν x λ

or, 3 Χ 10⁸ = 1.2 Χ 10^-10 ν
or, \(v=\frac{3 \times 10^8}{1.2 \times 10^{-10}}=2.5 \times 10^{18}\)

∴ Frequency of the X-ray is 5 x 110¹⁸ Hz

Question 2. The speed of light in vacuum is 3 x 10⁸ m/s What is the speed of light in a diamond? (The refractiveindex of a diamond is 2.4)

Answer: Speed of light in vacuum, c = 3 x 10⁸ m/s
Refractive index of diamond, µ = 2.4
∴ Speed of light in diamond

∴\(v=\frac{c}{\mu}=\frac{3 \times 10^8}{2.4}=1.25 \times 10^8 \mathrm{~m} / \mathrm{s}\)

Question 3. Speed of light in a medium is 2 x 10⁸ m/s. What is the refractive index of the medium?

Answer: Speed of light in medium, v = 2 x 10⁸ m/s
Speed of light in vacuum, c = 3 x 10⁸ m/s
∴ Refractive index of the medium, \(\mu=\frac{c}{v}=\frac{3 \times 10^8}{2 \times 10^8}=1.5\)

Miscellaneous Type Questions Match The Column

Question 1.

Column A (position of object)	Column B (position of image)
Centre of curvature	1. Behind the mirror
Between the focus and the pole	2. Between  the centre of curvature and infinity
At infinity	3. At the centre of curvature
Between the centre of curvature and the focus	4. At the focus

Answer:

Centre of curvature:  3. At the centre of curvature
Between the focus and the pole: 1. Behind the mirror
At infinity: 4. At the focus
Between the centre of curvature and the focus: 2. Between  the centre of curvature and infinity

Question 2.

Column A	Column B
Normal incidence on the mirror	1. Angular deviation is zero
Normal incidence at the surface separating water and glass	2. Angular deviation is 60°
In the case of refraction, i = 60°, r= 30°	3. Angular deviation is 180°.
In the case of reflection, the angle of incidence is 60°	4. Angular deviation is 30°

 

Answer:

Normal incidence on the mirror:  3. Angular deviation is 180°
Normal incidence at the surface separating water and glass:  1. Angular deviation is zero
In the case of refraction, i = 60°, r= 30°:  4. Angular deviation is 30°
In the case of reflection, the angle of incidence is 60°:  2. Angular deviation is 60°

WBBSE Solutions for Class 10 Physical Science and Environment

 

Organic Chemistry Topic A General Properties Of Organic Compounds Synopsis

WBBSE Class 10 Organic Chemistry Overview

1. In 1828, a German chemist Friedrich Wohler synthetically prepared urea, an organic compound found in the urine of most mammals, by heating an inorganic compound, ammonium .cyanate. This accidental synthesis gave a fatal blow to the ‘vital force theory’ in which it was considered impossible to prepare an organic compound in the laboratory from inorganic compounds.

2. The fundamental element of all organic compounds is carbon. Apart from carbon, H, O, N, S, P, halogens and some metals may also be present in organic compounds.
3. The self-linking property of carbon by virtue of which its atoms mutually combine with each other to form long open chains (straight or branched) and rings is called catenation. Using its catenation property, carbon forms a vast number of organic compounds.
4. Organic compounds in which all the adjacent carbon atoms are linked to one another by single bonds only are called saturated compounds. For example, methane, ethane.

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5. Organic compounds which contain at least one carbon-carbon double bond or triple bond are called unsaturated compounds. For example, ethylene, and acetylene.
6. Organic compounds containing carbon and hydrogen are called hydrocarbons.
7. An atom or a group of atoms present in an organic compound, which determines the nature and characteristic chemical properties of the compound is called a functional group.

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8. The phenomenon of existence of two or more compounds possessing the same molecular formula but different physical and chemical properties is called isomerism. Such compounds showing isomersim are called isomers.
9. The compounds having the same molecular formula but different structures are called structural isomers and the phenomenon is known as structural isomerism.

10. Structural isomerism may arise due to differences in functional groups or positions of the functional groups in the carbon chain.

ch₃ch₂ch₂oh Iupac Name

Example of functional group isomerism:

CH₃ — O — CH₃ (dimethyl ether)
CH₃ —  CH₂ — OH (ethanol)

Example of positional isomerism:

CH₃CH₂CH₂OH (propan-1-ol)

CH₃ — CH(OH) — CH₃ (propan-2-ol)

11. A homologous series may be defined as a series or group of similarly constituted organic compounds having same functional group and same general formula arranged in increasing order of molecular mass. Any two successive members of a homologous series differ in their molecular formula by ‘a’ CH₂ group or unit. The members of a homologous series are called homologues.

CH₃CH₂CH₂OH Iupac Name

12. General formula of the homologous series of alkanes is C_nH_{2n + 2} and the general formulae of alkene and alkyne are C_nH_2n-2 and C_nH_2n respectively.
13. A scientific and systematic approach of naming the organic compounds was adopted in a conference attended by chemists from worldwide. These rules of naming organic compounds are collectively known as IUPAC (International Union of Pure and Applied Chemistry) nomenclature of organic compounds. These rules have been modified from time to time.

Organic Chemistry Topic A General Properties Of Organic Compounds Short And Long Answer Type Questions

Question 1. Write three differences between organic and inorganic compounds.

Answer:

The major differences between organic and inorganic compounds are as follows:

Property	Organic compound	Inorganic compound
1. Constituents	All organic compounds must contain carbon. Apart from carbon, elements like H, N, 0, S, P, halogens etc., may also be present in organic compounds. Existence of more than 20 lakhs of organic compounds is known till date.	Inorganic compounds may or may not contain carbon. About 92 naturally occurring elements and some synthetically prepared elements form almost 90000 inorganic compounds which is far less than the number of organic compounds.
2. Melting point and boiling point	Organic compounds being covalent in nature generally have low melting and boiling points.	Most inorganic compounds being ionic in nature have high melting and boiling points.
3. Solubility	Organic compounds are soluble in organic and non-polar solvents like benzene, CCI4 etc., but insoluble in polar solvents such as water.	Inorganic compounds are generally soluble in polar solvents such as water but insoluble in non-polar solvents such as benzene.

 

Question 2. The general molecular formula isomers is C₄H₈. Predict the structural formula of those compounds.

Answer:

The general molecular formula of the isomers is C4H8. The formula conforms to the general formula CnH2n (where n = 4) which belongs to the alkene family.

Therefore, the possible isomers and their structural formulas are as follows:

Wbbse Class 10 Physical Science Solutions

 

 

 

Question 3. What is homologous series? Discuss the significance of homologous series.

Answer:

Homologous series

1. A homologous series may be defined as a series or a group of similarly constituted organic compounds having the same functional group and general formula arranged in order of their increasing molecular mass. Any two successive members of a homologous series differ in their molecular formula by a CH₂ -group or unit. The members of a homologous series are called homologues.

The significance of homologous series

2. If the method of preparation and properties of a certain member of a homologous series are known, then the method of preparation and properties of the other members of the same series can be easily predicted.

Wbbse Class 10 Physical Science Solutions

Question 4. Mention the important characteristics of a homologous series.

Answer:

The important characteristics of a homologous series

1. All the members of a homologous series have the same constituent elements and they can be represented by a single general formula.
2. If all the members of a homologous series are arranged in increasing order of their molecular masses, then two consecutive members of the family differ from one-another by one CH₂ -group and the difference in their molecular mass is 14 units.
3. All the members of a homologous series have almost similar chemical properties.

Question 5. How was Berzelius’ ‘vital force theory’ proved invalid?

Answer: In 1828, scientist Wohler accidentally prepared urea, an organic compound, by heating an inorganic compound, ammonium cyanate. Later, in 1845, Kolbe and in 1856, Berthelot prepared acetic acid and methane respectively from their constituent elements in the laboratory.

All these discoveries proved that no vital force inherent to living beings was involved in the formation of organic compounds. Thus, the ‘vital force theory’ was proved invalid.

Wbbse Class 10 Physical Science Solutions

Question 6. Why are organic compounds generally insoluble in water (a polar solvent)?

Answer: As organic compounds are formed by covalent bonds, they do not dissociate or ionise in water or any other polar solvents. Consequently, no electrostatic force of attraction develops between the water molecules and the molecules of the organic compound.

So, organic compounds are insoluble in water. However, some organic compounds dissolve in water by forming hydrogen bonds with water molecules (such as alcohols, glucose etc.) or by ionising in water (such as carboxylic acids).

Question 7. Why are the solutions of organic compounds generally non-conductor of electricity?

Answer: As organic compounds are formed by covalent bonds, they do not ionise in water or any other polar solvents. Hence, they cannot conduct electricity in solutions. However, aqueous solutions of carboxylic acids such as formic acid, acetic acid etc., ionise in water and conduct electricity to a small extent.

Understanding Organic Compounds and Their Properties

Question 8. What is the catenation property of carbon?

Answer:

The catenation property of carbon

The unique self-linking property of carbon due to which C-atoms mutually combine with each other by forming covalent single, double or triple bonds to form long open chains (straight or branched) and rings of different sizes is called catenation property of carbon. Due to this reason, carbon forms a vast number of organic compounds.

 

 

Question 9. Why is carbon able to show catenation property?

Answer: Due to the small size and moderate electronegativity of carbon atoms, it can form very strong and stable carbon-carbon covalent bonds. As a result, carbon atoms can link with one another by forming covalent single, double or triple bonds. For these reasons, carbon is able to show catenation property which leads to the formation of a large number of open chain and cyclic compounds.

Wbbse Class 10 Physical Science Solutions

Question 10. Briefly describe the tetrahedral model of the four valencies (bonds) of carbon as proposed by scientists van’t Hoff and Le Bel.

Answer: In 1874, scientists van’t Hoff and Le Bel proposed the tetrahedral model of the four valencies of carbon. According to this model—

1. All four valencies of a carbon atom cannot be on the same plane.
2. If a carbon atom is placed at the centre of an imaginary regular tetrahedron, then the four valencies of the C-atom will be directed towards the four corners of the tetrahedron.
3. The angle between any two valencies of the carbon atom is 109°28′.

 

 

Question 11. Describe the structure of ethane (C₂H₆) with the help of a suitable diagram.

Answer:

The structure of ethane (C₂H₆) with the help of a suitable diagram

In ethane (C₂H₆), the two adjacent C-atoms equally share one electron pair among themselves to form a single bond. Each of the C- atoms then satisfies its remaining three valencies by forming three single bonds with three hydrogen atoms.

Wbbse Class 10 Physical Science Solutions

 

 

Question 12. Describe the structure of ethylene (C₂H₄) with the help of a suitable diagram.

Answer:

The structure of ethylene (C₂H₄) with the help of a suitable diagram

In ethylene (C₂H₄), the two adjacent C-atoms share two electron pairs with each other to form a double bond and satisfy two of its valencies. Each of the two C-atoms then fulfils its remaining two valencies by forming single bond with two hydrogen atoms each. All the atoms in the molecule remain on the same plane.

 

 

Question 13. Describe the structure of acetylene (C₂H₂) with the help of a suitable diagram.

Answer:

The structure of acetylene (C₂H₂) with the help of a suitable diagram

In acetylene (C₂H₂), the two adjacent C-atoms share three electron pairs with each other to form a triple bond and satisfy three of its valencies. Each of the two C-atoms then fulfils its remaining valency by forming a single bond with one hydrogen atom each. All the atoms in the molecule remain in the same line and so, the molecule is linear in shape.

 

Question 14. What do you mean by functional group? Give one example.

Answer:

Functional group

An atom or a group of atoms present in an organic compound which determines the nature and characteristic chemical properties of the compound is called a functional group.

Example: —OH is the functional group of the alcohol series.

Question 15. Write the name and structure of the functional group of the alcohol series. Give an example of a compound containing that functional group.

Answer:

The name and structure of the functional group of the alcohol series.

The functional group of the alcohol series is the hydroxyl group and its structure is given by

2. A compound containing —OH as the functional group is ethyl alcohol (C₂H₅OH).

Question 16. Write the name and structure of the functional group present in aldehydes. Give an example of a compound containing that functional group.

Answer: The functional group present in aldehydes is the formyl or aldehydic group and its structure is given by:

 

 

A compound containing — CHO as the functional group is acetaldehyde (CH₃CHO).

Question 17. Write the name and structure of the functional group present in ketones. Give an example of a compound containing that functional group.

Answer: The functional group present in ketones is keto group and its structure is given by:

A compound containing     as the functional group is acetone (CH₃COCH₃)

Question 18. Write the name and structure of the functional group present in carboxylic acids. Give an example of a compound containing that functional group.

Answer: The functional group present in carboxylic acids is the carboxyl group and its structure is given by:

 

 

A compound containing — COOH as the functional group is acetic acid (CH₃COOH).

Types of Organic Reactions: Addition, Substitution, and Elimination

Question 19. Write the name and structure of the functional group present in amines. Give an example of a compound containing that functional group.

Answer: The functional group present in amines is the amino group and its structure is given by:

 

 

A compound containing -NH₂ as the functional group is methylamine (CH₃NH₂)

Question 20. Write the name and structure of the functional group present in ethers. Give an example of a compound containing that functional group.

Answer: The functional group present in ethers is the ether group and its structure is given by:

A compound containing — O — as the functional group is dimethyl ether (CH₃OCH₃).

Question 21. What is functional group isomerism? Give example.

Answer:

Functional group isomerism

The phenomenon of the existence of two or more compounds having the same molecular formula but different functional groups (i.e., belonging to different families) is called functional group isomerism.

Example: The functional groups in dimethyl ether (CH₃ — O — CH₃) and ethyl alcohol ( C₂H₅OH) are — O — and — OH respectively but both of these compounds have the same molecular formula of C₂H₆O.

Question 22. What is positional isomerism? Give example.

Answer:

Positional isomerism

The phenomenon of the existence of two or more compounds having the same structure of the carbon chain, i.e., the same carbon skeleton, but differing in the position of the multiple bond or functional group is called positional isomerism.

Example: The compounds, propan-1-ol or n- propyl alcohol (CH₃CH₂CH₂OH) and propan-2-ol or isopropyl alcohol [CH₃CH(OH)CH₃] both have the same molecular formula C₃H₈O, but the positions of — OH group is different in both the compounds.

Question 23. The members of a homologous series generally show similar chemical properties but different physical properties. Why?

Answer:

The members of a homologous series generally show similar chemical properties but different physical properties.

Due to the presence of the same functional group, the members of a particular homologous series show similar chemical properties. However, as the molecular masses of the members of a homologous series are different, the physical properties (such as melting point, boiling point, density etc.) dependent on the molecular mass of a compound are also different for the homologues.

Question 24. Only one alkyl group is formed from ethane whereas two different alkyl groups are formed from propane. Why?

Answer:

Only one alkyl group is formed from ethane whereas two different alkyl groups are formed from propane.

All the 6 H-atoms in an ethane molecule are equivalent. Hence, the removal of any one hydrogen atom from ethane produces the same alkyl group, i.e., the ethyl group ( — CH₂CH₃). On the other hand, two different types of hydrogen atoms are present in propane.

The two hydrogen atoms bonded to the central C-atom are different from the 6 H-atoms bonded to the terminal C-atoms in propane. So, the removal of the terminal H-atom produces an n-propyl group (CH₃CH₂CH₂—) whereas the removal of the non-terminal H-atom produces an isopropyl group

Question 25. Give the IUPAC names and structural formulas of the alkynes containing up to C-atoms.

Answer: HC = CH : ethyne
H₃C — C Ξ CH : propyne

Question 26. Give the IUPAC names and structural formulas of the alcohols containing up to 3 C-atoms.

Answer:

 

Question 27. Give the IUPAC names and structural formulas of the carbonyl compounds containing upto 3 C-atoms.

Answer:
Aldehydes:
HCHO: methanal
CH₃CHO : ethanal
CH₃CH₂CH₃CHO: propanal

Ketones:
CH₃COCH₃: propanone

Question 28. Give the IUPAC names and structural formulas of the carboxylic acids containing upto 3 C-atoms.

Answer:
HCOOH: methanoic acid
CH₃COOH: ethanoic acid
CH₃CH₂COOH : propanoic acid

Question 29. (1) Who proved that the main component of all organic compounds is carbon?
(2)All organic compounds contain carbon but all carbon compounds are not organic compounds. Explain

Answer:
(1) Scientist Lavoisier proved that the main component of all organic compounds is carbon.
(2) All carbon compounds are not organic compounds. Compounds like carbon monoxide, carbon dioxide, metallic carbonate, metallic bicarbonate, metallic cyanite etc. can not be termed as organic compounds as catenation property of carbon, isomerism etc. are not seen in these compounds. They all are inorganic compounds.

Question 30. What is a saturated compound? Give examples.

Answer:

Saturated compound

Organic compounds, in which all of the carbon atoms are attached through covalent single bonds are called saturated compounds.

Examples of saturated compounds are—

 

 

Question 31. What is an unsaturated compound? Give examples.

Answer:

Unsaturated compound

Organic compounds, in which at least two carbon atoms are attached through covalent double or triple bonds, are called unsaturated compounds.

Examples of unsaturated compounds are—

 

 

Question 32. What are alkenes or olefines? Give examples.

Answer:

Alkenes or olefines

Unsaturated hydrocarbons which contain at least one carbon-carbon double bond are called alkenes or olefins. The general formula of alkenes are C_nH_2n.

Examples: ethylene (CH₂=CH₂), propene (CH3 — CH = CH₂) etc.

Question 33. What are alkynes? Give examples.

Answer:

Alkynes

Unsaturated hydrocarbons which contain at least one carbon-carbon triple bond are called alkynes.

Examples: H — C Ξ C — H(acetylene),  CH3 — C=C-H(propyne)

Question 34. Differentiate between saturated and unsaturated hydrocarbons.

Answer:

Differences between saturated and unsaturated hydrocarbons are—

Topic	Saturated hydrocarbon	Unsaturated hydrocarbon
1. Nature of bond	Carbon atoms are attached through a covalent single bond.	At least two carbon atoms are attached through the double or triple bond.
2. Reactivity	Reactivity is comparatively less.	Chemical reactivity is comparatively more.
B. Nature of reaction	Usually participates in substitution reactions.	Usually participates in addition reactions.
4. Reaction with Br2 water	No reaction takes place. The red colour of bromine water remains unaltered.	An additional reaction takes place. The red colour of bromine water turns colourless.

 

Functional Groups in Organic Chemistry

Question 35. Why is C₂H₆ termed as saturated hydrocarbon but C₂H₄ is termed as unsaturated hydrocarbon?

Answer: All the carbon atoms of C₂H₆ are attached through the covalent single bond, hence it is termed as saturated hydrocarbon.

On the other hand, in C₂H₄, two adjacent carbon atoms are attached through a covalent double bond and hence it is termed as an unsaturated hydrocarbon.

Question 36. What do you mean by isomerism and isomer?

Answer:

Isomerism and isomer

1. The phenomenon of the existence of two or more compounds possessing the same molecular formula but different physical and chemical properties is known as isomerism.
2. Compounds with the same molecular formula but different physical and chemical properties, are termed as isomers to each other.

Question 37. Write down the definition and example of structural isomerism.

Answer:

Definition of structural isomerism

The phenomenon of existence of two or more compounds having the same molecular formula but different structures are called structural isomerism. For example, two structural isomers having the molecular formula C₂H₆O are—

H₃C — CH₂ — OH (ethanol) and H₃C — O —CH₃ (dimethyl ether)

Question 38. How would you identify two isomeric compounds denoted by the formula C₂H₆O? Or, Distinguish between ethanol and dimethyl ether by a chemical reaction.

Answer: Two compounds having same formula of C₂H₆O are dimethyl ether and ethanol. Ethanol reacts with sodium metal at room temperature, forming sodium ethoxide and H₂ gas. But dimethyl ether does not react with sodium.

2CH₃CH₂OH + 2Na → 2CH3CH₂ONa + H₂↑
CH₃ – O – CH₃ + Na → no reaction

Question 39. Write down the structural formula and IUPAC name of the following compounds:

1.CH₃CH₂CH₃
2. CH₂= CH₂
3.  H₃C-CH=CH₂
4. CH=CH
5.  CH₃-CH,CH
6. CH₃CH₂OH
7. CH₃CH(OH)CH₃
8. CH₃CH₂CH₂OH
9. CH₂(OH)CH(OH)CH₂(OH)
10. (CH₃)₃COH
11. C(CH₃)₃CHO
12. (CH₃)₂C= CH₂
13. CH₃COCH₃
14. CH₂(CI)CH₂(Cl)
15. CH₃CH(CI)CH₃

Answer:

Question 40. Write down the structural formula of—  

1. Ethanal
2. Methanoic acid
3. 2 -methyl propanal
4. 2,2-dichloro propane
5. Ethanoic acid
6. Ethanoic-1, 2-dioic acid
7. Tricloromethane
8. Methanol
9. Acetone

Answer:

 

Organic Chemistry Topic A General Properties Of Organic Compounds Answer In Brief

 

Question 1. Give the chemical equation for the reaction by which urea is produced from ammonium cyanate.
Answer: The chemical equation for the reaction by which urea is produced from ammonium cyanate is—

⇒ \(\underset{\text { (Ammonium cyanate) }}{\mathrm{NH}_4 \mathrm{CNO}} \frac{\text { heat }}{\text { (rearrangement) }} {\mathrm{NH}_2 \mathrm{CONH}_2}\)

Question 2. Give some examples of organic solvents.
Answer: Some examples of organic solvents are benzene, chloroform, acetone, ethanol etc.

Question 3. What are hydrocarbons?
Answer:

Hydrocarbons

The binary compounds of carbon and hydrogen are called hydrocarbons.

Question 4. What are saturated hydrocarbons?
Answer:

Saturated hydrocarbons

The hydrocarbons in which all the C-atoms are linked to one another through single covalent bonds only are called saturated hydrocarbons. For example, methane, ethane, and propane.

Question 5. What are unsaturated hydrocarbons?
Answer:

Unsaturated hydrocarbons

The hydrocarbons containing at least a double or a triple bond are called unsaturated hydrocarbons. For example, ethylene, and acetylene.

Question 6. Why are alkanes also known as paraffin?
Answer: Alkanes are also known as paraffin due to their less reactive nature.

Question 7. To which class do the organic compounds containing —the CHO group belong?
Answer: The organic compounds containing CHO as the functional group belongs to aldehyde.

Question 8. To which class do the organic compounds containing —CO— as the functional group belongs?
Answer: The organic compounds containing —CO — as the functional group belongs to the ketone.

Question 9. To which class do the organic compounds containing —COOH as the functional group belongs?
Answer: The organic compounds containing — COOH as the functional group belong to carboxylic acid.

Applications of Organic Chemistry in Daily Life

Question 10. To which class do the organic compounds containing —NH₂ as the functional group belongs?
Answer: The organic compounds containing —NH₂ as the functional group belongs to an amine.

Question 11. To which class do the organic compounds containing — O— as the functional group belongs?
Answer: The group name for organic compounds containing —O— as the functional group is ether.

Question 12. Name the first member of the alcohol homologous series. What is its IUPAC name?
Answer: The first member of the alcohol homologous series is methyl alcohol (CH₃OH).
Its IUPAC name is methanol.

Question 13. Name the first member of the aldehyde homologous series. What is its IUPAC name?
Answer: The first member of the aldehyde homologous series is formaldehyde (HCHO).
Its IUPAC name is methanal.

Question 14. Name the first member of the ketone homologous series. What is its IUPAC name?
Answer: The first member of the ketone homologous series is acetone (CH₃COCH₃).
Its IUPAC name is propanone.

Question 15. Name the first member of the carboxylic acid homologous series. What is its IUPAC name?
Answer: The first member of the carboxylic acid homologous series is formic acid (HCOOH).
Its IUPAC name is methanoic acid.

Question 16. Name the first member of the amine homologous series. What is its IUPAC name?
Answer: The first member of the amine homologous series is methylamine (CH₃NH₂).
Its IUPAC name is methenamine.

Question 17. Name the first member of the ether homologous series. What is its IUPAC name?
Answer: The first member of the ether homologous series is dimethyl ether(CH₃OCH₃).
Its IUPAC name is methoxymethane.

Question 18. What is isomerism?
Answer:

Isomerism

The phenomenon of the existence of two or more compounds having the same molecular formula but different chemical and physical properties is called isomerism.

Question 19. What do you mean by structural isomerism?
Answer:

Structural isomerism

The phenomenon of the existence of two or more compounds having the same molecular formula but different physical and chemical properties due to differences in their structures i.e., different bonding sequences is called structural isomerism.

Question 20. Between ethyl alcohol and dimethyl ether, which one does not react with metallic sodium?
Answer: Between ethyl alcohol and dimethyl ether, the latter does not react with metallic sodium.

Question 21. What is the minimum number of C-atoms required to represent the structural formula of a ketone?
Answer: A minimum of three C-atoms are required to represent the structural formula of a ketone.

Question 22. Name the two scientists who proved ‘the vital force theory’ wrong successfully synthesising acetic acid and methane in the laboratory.
Answer: Kolbe and Berthelot were the scientists who proved ‘the vital force theory’ to be wrong.

Question 23. Which two alkyl groups are formed when one hydrogen atom is removed from propane?
Answer: The two alkyl groups formed are—the propyl group (CH₃CH₂CH₂ —) and the isopropyl group (H₃C — CH — CH₃).

Word Problems on Organic Reactions with Solutions

Question 24. Why is ethanol called ‘grain alcohol’?
Answer: Ethanol is also called ‘grain alcohol’ because it can be prepared by the fermentation of finely ground grains (such as corn).

Question 25. Which type of chemical bond is present in organic compounds?
Answer: Covalent bond.

Question 26. Give an example of a biomolecule.
Answer: Glucose.

Question 27. State whether the organic compound of molecular formula C₄H₈ is saturated or unsaturated?
Answer: C₄H₈ (C_nH_2n, where n = 4) is an alkene compound, hence it is unsaturated in nature.

Question 28. Write down the number of H-atoms present in 4C-alkane.
Answer: The number of H-atoms in a 4C-alkane is 10.

Question 29. What is the number of H-atoms in a 3- carbon alkyne?
Answer: The number of ‘H’-atoms in a 3-carbon alkyne is 4.

Question 30. Name the simplest alkyne.
Answer: Ethyne (H — C = C — H).

Question 31. Which type of hydrocarbon is acetylene?
Answer: Acetylene is an unsaturated hydrocarbon.

Question 32. What is the H — C—H bond angle in methane molecules?
Answer: 109°28′

Question 33. How many chemical bonds are there in ethylene?
Answer: 6 chemical bonds are present in ethylene.

Question 34. Write down the H —C —H bond angle in ethylene.
Answer: The h — C—H bond angle in ethylene is 120°

Question 35. Give an example of a non-carbon functional group.
Answer: Hydroxyl ( — OH).

Question 36. Which type of isomerism can be seen in propanone and propanal?
Answer: Functional group isomerism.

Question 37. Write down the formula of functional group isomer of CH₃CH₂OH.
Answer: CH₃ — O — CH₃ (dimethyl ether)

Question 38. Write down the structural formula of positional isomer of CH₃CH₂CH₂OH.
Answer: Equation

Question 39. Write down the next two higher homologue of methane.
Answer: C₂H₆ (ethane) and C₃H₈  (propane).

Question 40. Write down the formula of first alkene homologue.
Answre: CH₂ = CH₂ (ethene)

Question 41. Write down the next homologue of C₂H₅OH.
Answer: The next homologue of C₂H₅OH is propanol (C₃H₇OH).

Question 42. Write down the full form of IUPAC.
Answer: International Union of Pure and Applied Chemistry.

Question 43. Write down the formula of prop-1-ene.
Answer: Formula of prop-1-ene is H₂C = CH — CH₃.

Question 44. Write down the IUPAC name of HC = CH.
Answer: IUPAC name of HC ≡ CH is ethyne.

Question 45. Write down the IUPAC name of CH₃CH₂OH.
Answer: ethanol.

Question 46. Write down the IUPAC name of CH₃CH₂CH₂OH.
Answer: propan-1-ol.

Question 47. Write down the IUPAC name of CH₃CH₂CHO.
Answer: propanal.

Question 48. Write down the IUPAC name of CH₃CH₂COOH.
Answer: propanoic acid.

 

Organic Chemistry Topic A General Properties Of Organic Compounds Fill In The Blanks

 

Question 1. Organic compounds are generally insoluble in ______ solvents.
Answer: Polar

Question 2. Most organic compounds are combustible due to the presence of _____ and _______.
Answer: Carbon, hydrogen

Question 3. Organic compounds do not ______ when dissolved in suitable solvents.
Answer: Ionise

Question 4. A carbon atom forms _______ bond with other atoms.
Answer: Covalent

Question 5. In most cases/carbon cannot form _____ compounds.
Answer: Ionic

Question 6. In the tetrahedral model of C-atom, all the four bonds of carbon atom does not remain on the ______ plane.
Answer: Same

Question 7. In the tetrahedral arrangement of four valencies of carbon, the carbon atom is present at the ______ of the tetrahedron and the four bonds are directed towards the four ______ of the tetrahedron.
Answer: Center, Corners

Question 8. The large number of organic compounds exists due to _____ and______ Of carbon.
Answer: Catenation, Isomerism

Question 9. The chain formed by carbon atoms due to catenation may be either ______ or ______
Answer: Open, Cyclic

Question 10. In methane, all the four valencies of carbon atom are satisfied by ______
Answer: Single, Bonds

Question 11. The ______ of an organic compound determines its chemical property.
Answer: Functional group

Question 12. The chemical reactivity of saturated hydrocarbons is very _____ while that of unsaturated hydrocarbons is quite ______
Answer: Less, high

Question 13. Between alcohols and ethers, _______ are relatively less reactive.
Answer: Ethers

Question 14. Between aldehydes and ketones, ______ are chemically more reactive.
Answer: Aldehyde

Question 15. If an alcohol and an ether are isomers, then it is an example of ______ isomerism.
Answer: Functional group

Question 16. Two adjacent members of a homologous series differ in their molecular formula by a _____ group.
Answer: CH₂

Question 17. All the compounds of a homologous series contain the same _____
Answer: Functional group

Question 18. The chemical properties of all the members of a homologous series is generally_________
Answer: Identical

Question 19. The phenomenon of similarity in the properties of the members of a homologous series is known as ________
Answer: Homology

Question 20. When one hydrogen atom is removed from ethane, _______ group is formed.
Answer: Ethyl

Question 21. The IUPAC name of the carboxylic acid having lowest molecular mass is _______
Answer: Methanoic acid

Question 22. The IUPAC name of the aldehyde containing two C-atoms is ______
Answer: Ethanal

Question 23. Covalent compounds are usually _________ in water.
Answer: Insoluble

Question 24.Different compounds with same molecular formula are ______ to each other.
Answer: Isomers

Question 25. n -propyl alcohol and isopropyl alcohol are _______ isomers.
Answer: Positional

 

Organic Chemistry Topic A General Properties Of Organic Compounds State Whether True Or False

 

Question 1. The first organic compound to be synthesised from an inorganic compound is acetic acid.
Answer: False

Question 2. Methane and ethane are saturated hydrocarbons while ethylene and acetylene are unsaturated hydrocarbons.
Answer: True

Question 3. In C₂H₂, the two adjacent carbon atoms are linked by a double bond.
Answer: False

Question 4. Alkenes are also called olefins because the lower members of this family react with halogens to form oily substances.
Answer: True

Question 5. Ethylene reacts with sulphur monochloride ( S₂CI₂) to produce mustard gas.
Answer: True

Question 6. C_nH_2n+1OH is the general formula of carboxylic acids.
Answer: False

Question 7. 2-bromopentane and 3-bromopentane are positional isomers.
Answer: True

Question 8. The molecular formula, C₃H₆O, represents two functional isomers namely, acetone and propionaldehyde.
Answer: True

Question 9. Thermal decomposition of organic compounds is known as pyrolysis.
Answer: True

Question 10. The first member of the aldehyde family is formaldehyde and its IUPAC name is ethanal.
Answer: False

Question 11. The IUPAC name of dimethyl ether is methoxymethane.
Answer: True

Question 12. Aldehydes and ketones are collectively known as carbonyl compounds.
Answer: True

Question 13. Ethane can decolourise the red colour of Br₂ dissolved in carbon tetrachloride solution.
Answer: True

Question 14. If one hydrogen atom is removed from propane, then two alkyl groups are formed namely, propyl and isopropyl.
Answer: True

Question 15. All the organic compounds are made of carbon.
Answer: True

Question 16. Catenation is the property of organic compounds.
Answer: False

Question 17. Formula of the 3-carbon alkane is C₃H₆.
Answer: False

Question 18. H —C —H bond angle in methane is 120°.
Answer: False

Question 19. C₂H₆ is a saturated hydrocarbon.
Answer: True

Question 20. Number of H-atoms in a 2-carbon alkane is 5.
Answer: False

Question 21. Functional group of alcohol is —OH .
Answer: True

Question 22. Common name of methanal is formaldehyde.
Answer: True

Question 23. IUPAC name of formic acid is methanoic acid.
Answer: True

Organic Chemistry Topic B Methane, Ethylene, Acetylene, LPG And CNG Synopsis

 

1. Methane (CH₄), ethylene (C₂H₄) and acetylene (C₂H₂) are the first members of alkane, alkene and alkyne homologous series respectively. These compounds have extensive applications in different industries.
2. The gaseous component obtained at the time of refining crude petroleum by fractional distillation is liquefied under high pressure and stored in steel cylinders. This is known as LPG (Liquefied Petroleum Gas). It is widely used as a fuel.

3. The gas trapped in petroleum mines or coal mines is called natural gas. It is liquefied under high pressure and is known as CNG (Compressed Natural Gas). It is also used as a fuel. The main component of CNG is methane (~ 90%). Ethane, ethene, propane, butane and little amount of pentane is generally present in CNG, in addition to methane.

4. Methane is a saturated hydrocarbon belonging to the alkane family. It mainly participates in substitution reactions.

5. Combustion of methane: in presence of air, methane burns in a blue flame to form carbon dioxide and water.

⇒ \(\begin{array}{r}
\mathrm{CH}_4+2 \mathrm{O}_2 \rightarrow \mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O}+\text { heat } \\
(213 \mathrm{kcal} / \mathrm{mol})
\end{array}\)

6. Ethylene is an unsaturated hydrocarbon belonging to the alkene family. It mainly participates in addition reactions. Polythene, a polymer of ethylene, has many practical applications.

7. Addition reaction of ethylene: When ethylene gas is passed through Br₂-solution (in CCI₄,CHCI₂), the red colour of the solution disappears and ethylene dibromide is formed.

8. Acetylene is an unsaturated hydrocarbon of the alkyne family and generally participates in addition reactions.

9. Uses of acetylene:
(1)Used to form oxyacetylene flame (3000°C)
(2)Used to form artificial rubber, artificial fibre, to lit carbide lamp, to form PVC etc.

9. Addition reaction of acetylene:
(1)Acetylene may be hydrogenated in presence of Ni- catalyst at 200°C, or in presence of Pt, Pd or Raney Ni catalyst at room temperature, to form ethylene and then ethane.

⇒ \(\begin{aligned}
& \mathrm{H}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H} \underset{2}{200^{\circ} \mathrm{C}} \mathrm{H}_2 / \mathrm{Ni}=\mathrm{CH}_2 \frac{\mathrm{H}_2 / \mathrm{Ni}}{200^{\circ} \mathrm{C} \downarrow} \\
& \mathrm{H}_3 \mathrm{C} \equiv \mathrm{CH}_3
\end{aligned}\)

(2)Acetylene decolourises the red colour of bromine-water as well as of liquid bromin.

⇒  \(\begin{array}{r}
\mathrm{H}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}+\mathrm{Br}_2 \frac{\mathrm{H}_2 \mathrm{O}}{\longrightarrow} \mathrm{HCBr}=\mathrm{CHBr} \\
\text { (red). } \\
\begin{array}{c}
1,1,2,2 \text {-dibromoethene } \\
\text { (colourless) }
\end{array}
\end{array}\)

⇒  \(\begin{aligned}
& \mathrm{H}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}+2 \mathrm{Br}_2 \stackrel{\mathrm{H}_2 \mathrm{O}}{\longrightarrow} \mathrm{HCBr}_2-\mathrm{CHBr}_2 \\
& \text { (red) 1,1,2,2-tetrabromoethane } \\
& \text { (colourless) } \\
&
\end{aligned}\)

 

Organic Chemistry Topic B Methane, Ethylene, Acetylene, LPG and CNG Short And Long Answer Type Questions

Question 1. What are the important uses of methane?

Answer:

Important uses of methane

1. ‘Carbon black’ obtained due to cracking of methane is used in making black paint, printing ink and in rubber industries for making motor tyres.
2. Important chemical compounds such as methyl chloride, acetylene, formaldehyde, methanol are prepared from methane.
3. Due to high calorific value of methane (1000 Btu/ft³), it is used as a fuel.

Question 2. Mention some important uses of ethylene.

Answer:

Some important uses of ethylene

1. Ethylene is used in ripening and preservation of fruits.
2. It is used in the preparation of ethylene dichloride, ethylene dibromide, mustard gas (toxic in nature) etc.
3. It is used in the manufacture of plastics such as polythene, polystyrene etc., and synthetic fibres like nylon, terylene etc.
4. A mixture of 80% ethylene and 20% oxygen is used as an anesthetic in surgeries.

Question 3. Mention some important uses of acetylene.

Answer:

Some important uses of acetylene

1. Acetylene is used for producing oxy-acetylene flame, used for cutting steel and other metals.
2. It is used to produce bright illuminating flame in carbide lamps.
3. It is used in the industrial preparation of acetaldehyde, acetic acid, ethyl alcohol, acetone etc.
4. Acetylene is also used in the manufacture of industrial non-inflammable solvents like acetylene tetrachloride (westron, C₂H₂CI₄) and trichloroethylene (westrosol, CI₂C = CHCI) used in dissolution of fats, oil and resins.

 

 

Question 4. How will you distinguish between the following pairs of compounds? Ethylene and acetylene, ethane and acetylene, ethane and ethylene.

Answer:

Reagent	Ethylene	Acetylene
Ammoniacal Cu2CI2 solution	No precipitate is observed.	A red precipitate is formed.

 

Reagent	Ethane	Acetylene
Br2/CCI4 solution	No change in the colour of bromine solution.	The orange-brown colour or bromine solution disappears.

 

Reagent	Ethane	Ethylene
Br2/CCI4 solution	No change in the colour of bromine solution.	The orange-brown colour of bromine solution disappears.

Question 5. Write some important uses of LPG.

Answer:

Some important uses of LPG

1. As the calorific value of LPG is very high (29500 kcal/m³)it is used as a fuel for cooking purposes.
2. It is also used as a fuel in vehicles and industries.
3. As boiling point of LPG is quite low, it is also used as a refrigerant.

Question 6. What are the advantages of using LPG as a fuel?

Answer:

The advantages of using LPG as a fuel

1. As the calorific value of LPG is very high, it easily generates large amount of heat which is used up in heating other objects.
2. LPG undergoes complete combustion. So, it does not produce ashes or liberate poisonous gas such as CO during its combustion.
3. Using a regulator, the flow of LPG from the cylinder can be controlled. Hence, its combustion can be controlled depending upon the requirement of heat.
4. LPG cylinders can be conveniently transported to far off places as well.
5. It does not contain carbon monooxide and hence it is not harmful.

Question 7. Which sulphur compound is mixed with LPG? Why is this compound added?

Answer:
1. Ethyl mercaptan (C₂H₅SH) is mixed with LPG. It emits a very foul smell.
2. The gases constituting LPG are colourless and odourless but are highly imflammable. So, if these gases accidentally leak out of the cylinder, it is not possible to detect them which may lead to fatal accidents. So, as a precautionary measure, ethyl mercaptan is added to LPG as its characteristic foul smell help to detect any gas leakage thereby preventing accidents.

Question 8. Mention some important uses of CNG. Mention one environmental disadvantage of using CNG.

Answer:

Some important uses of CNG.

1. As the calorific value of CNG is very high (21300 Btu/lb), it is widely used as a fuel.
2. CNG causes least air pollution when it is used in vehicles. So, nowadays CNG is extensively used as a fuel in public transport like buses, autos, taxis etc.

Small amount of methane is released into the atmosphere due to the usage of CNG. Methane being a greenhouse gas causes global warming.

Question 9. What-are the advantages of using CNG as a fuel?

Answer:

The advantages of using CNG as a fuel

1. The carbon content of CNG is very low. So, it produces very less amount of carbon particles and CO₂ compared to other fuels during its combustion. Thus, CNG causes less air pollution.
2. It has a very high ignition temperature (1350°F). Hence, CNG does not burn easily which makes it safe to use.
3. No poisonous gas and ash are produced during combustion of CNG.
4. The calorific value of CNG (21300 Btu/ lb) is higher than that of diesel and LPG.

Question 10. What are the disadvantages of using CNG as a fuel?

Answer:

The disadvantages of using CNG as a fuel

1. As it is a gaseous fuel, the amount of heat generated by CNG per unit volume is low compared to other fuels.
2. The tanks required to store CNG are usually 3 to 5 times larger than those required for other fuels like diesel.
3. CNG being lighter than air- rises up and rapidly spreads in the atmosphere when it comes out of a cylinder. Consequently, the entire place gets covered by CNG leading to serious explosion.
4. During combustion of CNG, small amount of methane which is a greenhouse gas, is released into the atmosphere.

Question 11. Mention the industrial sources of methane ( CH₄).

Answer:

The industrial sources of methane ( CH₄)

1. The natural gas obtained from petroleum mines contains large amount of methane (40-90%).
2. Coal gas contains almost 40% (by volume) methane.
3. Methane is found in traces in the gas obtained from coal mines.
4. A mixture of CO₂ and H₂ or CO and H₂ when passed over hot Ni at 250-400°C, methane is produced.

⇒ \(\mathrm{CO}_2+4 \mathrm{H}_2 \underset{250-400^{\circ} \mathrm{C}}{\stackrel{\text { Ni-dust }}{\longrightarrow}} \mathrm{CH}_4+2 \mathrm{H}_2 \mathrm{O} \text {; }\)

⇒ \(\mathrm{CO}+3 \mathrm{H}_2 \underset{250-400^{\circ} \mathrm{C}}{\stackrel{\text { Ni-dust }}{\longrightarrow}} \mathrm{CH}_4+\mathrm{H}_2 \mathrm{O}\)

Question 12. Mention the major industrial sources of ethylene (C₂H₄).

Answer:

The major industrial sources of ethylene (C₂H₄)

1. The natural gas obtained from petroleum mines contains almost 20% of ethylene.
2. Ethylene is found in large volumes in coke oven gas.
3. Coal gas contains small amount of ethylene (almost 4%).
4. During cracking (the process by which long-chain hydrocarbons are degraded into smaller hydrocarbons by applying heat) of petroleum, ethylene is produced as a by product.

Question 13. Mention the major indu acetylene (C₂H₂).

Answer:

The major indu acetylene (C₂H₂)

1. Coal gas contains traces of acetylene (almost 0.06%)
2. Acetylene is produced by decomposing natural gas (mainly methane) at high temperature.

Question 14. What is LPG? Mention its industrial source?

Answer:

LPG:

1. LPG stands for Liquefied Petroleum Gas. It is a mixture of hydrocarbons of low molecular mass (upto 3-4 C-atoms). The major constituents of LPG are n-butane, propane, isobutane, butene etc. It also contains small amounts of propylene and ethane.
2. LPG is obtained by compressing the crude petroleum during refining into liquid under high pressure. It is stored in steel cylinders.

Question 15. Mention the constituents and industrial sources of CNG.

Answer:

The constituents and industrial sources of CNG

Constituents: The major constituent of CNG (Compressed Natural Gas) is methane (almost 90%). Apart from this, trace amounts of ethane, ethene, propane, butane and low-boiling pentane are also present in CNG.

Industrial sources: Natural gas is obtained above petroleum in petroleum mines and also from the coal mines. This gas is compressed into liquid by applying high pressure.

Question 16. Write with equation what happens when methane reacts with chlorine gas in presence of diffused sunlight.

Answer: In presence of diffused sunlight, methane undergoes substitution reaction with chlorine. In this case, the hydrogen atoms of methane are successively replaced by chlorine atoms one after another to form methyl chloride, methylene chloride, chloroform and carbon tetrachloride.

CH₄ + Cl₂ → CH₃CI (methyl chloride) + HCI
CH₃CI + Cl₂ → CH₂CI₂ (methylenechloride) + HCI
CH₂CI₂ + Cl₂ → CHCI₃ (chloroform) + HCI
CHCI₃ + Cl₂ → CCI₄ (carbon tetrachloride) + HCI

Question 17. Write down the hydrogenation reaction of ethylene specifying suitable conditions and chemical equation.

Answer: Hydrogen adds to the double bond of ethylene molecule at ordinary temperature and pressure in the presence of finely divided platinum or palladium or Raney nickel or at a temperature of about 200-300°C in the presence of finely divided nickel catalyst to produce ethane.

 

 

Question 18. What happens when ethylene gas is passed through bromine dissolved in carbon tetrachloride or in chloroform? Give equation. Or, How will you prove that ethylene is an unsaturated compound? Or, Give an example of addition reaction of ethylene.

Answer: When ethylene gas is passed through bromine dissolved in carbon tetrachloride ( CCI₄) or in chloroform (CHCI₃), the orange-brown colour of the solution is decolourised due to the formation of a colourless compound, ethylene dibromide or 1,2-dibromoethane. This reaction proves that ethylene is an unsaturated compound.

 

Question 19. Write a short note on polymerisation of ethylene.

Answer:

Polymerisation of ethylene

When ethylene is heated at 150 – 200°C in the presence of oxygen or peroxide catalyst under a very high pressure (1500-2000 atm), a large number of ethylene molecules combine with each other to form a solid compound of high molecular mass (approx. 20000) called polyethylene or polythene. This reaction is known as polymerisation of ethylene.

 

 

Question 20. Write down the hydrogenation reaction of acetylene specifying the suitable conditions and chemical equation.

Answer: Hydrogen reacts with acetylene at ordinary temperature in presence of Raney nickel or powdered platinum or palladium catalyst or at a temperature of about 200 – 300°C in presence of powdered nickel catalyst to produce ethane. The reaction occurs in two steps. In the first step, one molecule of hydrogen adds to acetylene to form ethylene and in the second step, another molecule of hydrogen adds to ethylene to form ethane.

Question 21. Which of the following hydrocarbons will participate in addition reactions— C₄H₁₀ ,C₃H₄ ,C₃H₆

Answer: Only unsaturated hydrocarbons participate in addition reactions. Among the given hydrocarbons, C₃H₆ (an alkene) and C₃H₄ (an alkyne) are unsaturated compounds and hence, undergo addition reactions. On the other hand,  C₄H₁₀ (an alkane) being a saturated compound does not undergo addition reaction.

Wbbse Class 10 Organic Compounds

Question 22. How will you prove that acetylene is an unsaturated compound? Or, What happens when an excess amount of acetylene gas is passed through bromine? Give equation.

Answer: When acetylene gas is passed through orange- brown coloured bromine water, 1,2- dibromoethene or acetylene dibromide is formed and consequently, the colour of bromine water is discharged. This reaction proves that acetylene is an unsaturated compound.

HC = CH + Br₂ → CHBr = CHBr
(orange-brown solution)  1,2-dibromo ethene (colourless)

Question 23. Three cylinders separately contain methane, ethylene and acetylene. How will you identify the gases?

Answer:

Three cylinders separately contain methane, ethylene and acetylene.

At first, the three gases are separately passed through ammoniacal cuprous chloride solution. The gas which produces a red precipitate is acetylene. After that, the two remaining gases are separately passed through bromine dissolved in CCI₄ solution. The gas which turns orange-brown solution of bromine colourless is ethylene. Thus, the remaining gas is definitely methene.

Question 24. what happens when methane is combusted in the presence of oxygen? Give balanced equations.

Answer: Methane burns with a light blue flame in excess air or oxygen and produces carbon dioxide, water vapour and huge heat is evolved.

⇒ \(\mathrm{H}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H} \stackrel{\mathrm{CuCl}_2}{\longrightarrow} \mathrm{H}-\underset{\text { red ppt }}{\mathrm{C} \equiv \stackrel{\ominus}{\mathrm{C}} \mathrm{Cu}^{\oplus} \downarrow}\)

Question 25. What is Will-o’-the-wisp?

Answer:

Will-o’-the-wisp

Methane is formed in marshy lands due to the bacterial decomposition of organic matter. Moreover, phosphine (PH₃) and phosphorous tetrahydride (P₂H₄) are also formed as a result of the putrefaction of animal bodies. P₂H₄ rapdily burns in the air.

So when the mixture of CH₄, PH₃ and P₂H₄ comes in contact with air, P₂H₄ sets the gas mixture on fire and the heat produced causes methane to burn with a light blue flame. As a result, an intermittent source of light is produced. This is known as ‘Will-o’-the-wisp’.

Wbbse Class 10 Organic Compounds

Question 26. How will you prepare ethylene from acetylene?

Answer: In the presence of Lindler’s catalyst, acetylene combines with only one molecule of hydrogen to form ethylene.

⇒ \(\mathrm{HC} \equiv \mathrm{CH}+\mathrm{H}_2 \frac{\mathrm{Pd}-\mathrm{CaCO}_3 / \mathrm{Pb}(\mathrm{OAc})_2}{\text { Lindler’s catalyst }} \mathrm{H}_2 \mathrm{C}=\mathrm{CH}_2)\)

Question 27. A’ and ‘B’ are two unsaturated hydrocarbons with two carbon atoms in each. In reaction with bromine, each molecule of ‘A’ combines with 1 molecule of bromine and while each molecule of ‘B’ combines with 2 molecules of bromine. Write down the structure of ‘A’ and ‘S’. Write down the balanced equation for the reaction of ‘6’ with bromine.

Answer: Unsaturated hydrocarbons with two carbon atoms are ethylene and acetylene. Since each molecule of ‘A’ combines with 1 molecule of bromine, therefore ‘A’ is ethylene (CH₃ = CH₂). Again since each molecule of ‘6’ combines with 2 molecules of bromine, therefore ‘S’ is acetylene (HC ≡ CH).

Visual Representation of Organic Molecules

∴

 

Organic Chemistry Topic B Methane, Ethylene, Acetylene, LPG and CNG Answer In Brief

 

Question 1. What is carbon black?
Answer:

Carbon black

At 1000°C, methane decomposes to form fine particles of carbon which is known as carbon black.

⇒ \(\mathrm{CH}_4 \stackrel{1000^{\circ} \mathrm{C}}{\longrightarrow} \mathrm{C}+2 \mathrm{H}_2 \uparrow\)

Question 2. What are the uses of carbon black?
Answer: Carbon black is used in the manufacture of tyres, typewriter ribbons, printing ink, shoe polish etc.

Question 3. Which member of the alkane homologous series is the major constituent of natural gas?
Answer: Methane (CH₄) belonging to the alkane homologous series is the major constituent of natural gas.

Question 4. Give an example of a hydrocarbon which is also a greenhouse gas.
Answer: Methane (CH₄) is a hydrocarbon as well as a greenhouse gas.

Wbbse Class 10 Organic Compounds

Question 5. Which hydrocarbon is used in the industrial preparation of formaldehyde?
Answer: Methane (CH₄) is used in the industrial preparation of formaldehyde.

Question 6. Which hydrocarbon is used in the manufacture of industrial non-inflammable solvents namely Westron and Westrosol (used to dissolve fats, oils, resins etc.)?
Answer: Acetylene (C₂H₂) is used in the manufacture of industrial non-inflammable solvents namely Westron and Westrosol.

Question 7. Write with equation what happens when methane undergoes combustion in the presence of sufficient oxygen.
Answer: When methane undergoes combustion it burns with a pale bluish non-luminous flame in presence of sufficient oxygen or air to produce CO₂ and water vapour.

⇒ \(\begin{array}{r}
\mathrm{CH}_4+2 \mathrm{O}_2 \rightarrow \mathrm{CO}_2+2 \mathrm{H}_2 \mathrm{O}+\text { heat } \\
\left(213 \mathrm{kcal} \cdot \mathrm{mol}^{-1}\right)
\end{array}\)

Question 8. Name the catalyst used in the addition reaction between ethylene and hydrogen.
Answer: Pt, Pd or Raney nickel is used as the catalyst in the addition reaction between ethylene and hydrogen.

Question 9. Under what conditions do the H-atoms of methane gets replaced by chlorine atoms in stepwise substitution reaction?
Answer: When chlorine reacts with methane in the presence of diffused sunlight the H-atoms of methane gets successively replaced by chlorine atoms in stepwise substitution reaction.

Question 10. What is the main component of marsh gas?
Answer: Main component of Marsh gas is methane (CH₄).

Question 11. Write down the formula of the organic compound formed in the first step of the substitutional reaction of methane with chlorine.
Answer: Formula of the compound is CH₃CI (methyl chloride).

Question 12. Which gas is responsible for Will- ‘O’-the wisp?
Answer: Methane gas (CH₄) along with phosphine (PH₃) and diphosphane (P₂H₄) are responsible for Will- ‘O’-the wisp.

Wbbse Class 10 Organic Compounds

Question 13. Which one undergoes addition reaction, saturated or unsaturated hydrocarbon?
Answer: Unsaturated hydrocarbon undergoes addition reaction.

Question 14. How will you identify wheather an organic compound is unsaturated or not?
Answer: If the red colour of bromine water decolourises in reaction with the sample organic compound, it can be said that the sample organic compound is unsaturated.

Question 15. CaC₂ + 2H₂O → A + Ca(OH)2; Identify ‘A’.
Answer: ‘A’ is acetylene, HC = CH.

Question 16. Write down the name of the compound formed when acetylene gas is passed through red coloured bromine water.
Answer: 1,2-dibro methene (CHBr = CHBr)

Question 17. What is the main component of LPG?
Answer: Butane (C₄H₁₀).

Question 18. Mention the use of LPG.
Answer: LPG is used as fuel in cooking and industrial purpose.

Question 19. Which bad-smelling component is mixed with LPG?
Answer: Ethyl mercaptan ( C₂H₅SH)

Question 20. What is the industrial source of CNG?
Answer: The source is usually shale rock far beneath the earth’s surface. Again the trapped gas in petroleum mines is CNG.

Question 21. Which one of petrol and CNG causes comparatively lower air pollution when used as fuel for vehicles?
Answer: CNG causes comparatively lower pollution.

Question 22. Mention the use of CNG.
Answer: CNG is used as fuel in bus, taxi, auto rickshaws etc.

Organic Chemistry Topic B Methane, Ethylene, Acetylene, LPG and CNG Fill In The Blanks

Physical Science Class 10 West Bengal Board

Question 1. The characteristic odour of LPG is due to the presence of _______
Answer: Ethyl mercaptan

Question 2. Methane gas is used as a fuel because of its high ______
Answer: Calorific value

Question 3. At high temperatures, natural gas is decomposed to produce _______
Answer: Acetylene

Question 4. _______ is formed when all the hydrogen atoms in methane are replaced by chlorine atoms.
Answer: Carbon tetrachloride

Question 5. The number of organic compounds formed in the reaction between methane and chlorine is _______
Answer: 4

Question 6. The temperature of the oxy-acetylene flame is ______°C.
Answer: 3000

Question 7. The major constituent of LPG is ______
Answer: Butane

Question 8. To reduce air pollution, _______ is used as an alternative fuel nowadays in buses, taxis, auto-rickshaws and other vehicles.
Answer: CNG

Question 9. In the presence of excess oxygen, methane burns with a non-luminous, pale _____ flame.
Answer: Blue

Question 10. In addition reaction with hydrogen, each molecule of ethylene combines with _____ molecule of hydrogen.
Answer: One

Wbbse Class 10 Organic Compounds

Question 11. At normal temperature, ethylene is a while polythene is a ______ while polythene is a ______
Answer: Gas, solid

Question 12. The component of Marsh gas which ignites in contact with air is ______
Answer: P₂H₄

Question 13. 3’H’ atoms of methane when substituted by 3’CI’ atoms, ______ is formed.
Answer: Chloroform

Question 14. Full form of LPG is ______
Answer: Liquefied Petroleum Gas

Question 15. Full form of CNG is ______
Answer: Compressed Natural Gas

 

Organic Chemistry Topic B Methane, Ethylene, Acetylene, LPG and CNG State Whether True Or False

Physical Science Class 10 West Bengal Board

Question 1. Methane usually participates in additional reactions.
Answer: False

Question 2. A mixture of methane and oxygen explodes when it comes in contact with fire.
Answer: True

Question 3. The decomposition of acetylene at a temperature of about 1000°C produces fine particles of carbon called carbon black.
Answer: False

Question 4. LPG stands for Liquid Petroleum Gas.
Answer: False

Wbbse Class 10 Organic Compounds

Question 5. Acetylene is used to produce illuminating flame in carbide lamps.
Answer: True

Question 6. 10% methane is present in coal gas (by volume).
Answer: False

Question 7. The smell of H₂S can be smelled in leakage of the LPG cylinder.
Answer: False

Question 8. CNG produces comparatively lower pollutants than LPG.
Answer: True

 

Organic Chemistry Topic C Polymer, Ethyl Alcohol, Acetic Acid And Denatured Spirit Synopsis

 

1. A chemical reaction in which a number of simple molecules react to form a larger molecule of high molecular mass is called a polymerisation reaction. The large molecule formed in the reaction is called polymer and the small molecules forming the repeating units in the polymer are called monomers. For example, polyethene, PVC, and Teflon.

2. Non-biodegradable synthetic polymers create different environmental hazards. As the manufacture of biodegradable polymers is still not cost-effective yet, use of natural polymers such as cotton, jute, wood, paper etc., should be encouraged.
3. Ethyl alcohol, acetic acid and denatured spirit have different commercial utilities.

Wbbse Class 10 Physical Properties Of Organic Compounds

4. Uses of acetic add:
(1)Acetic acid is used to prepare white lead, and acetone. 0 5-8% aqueous solution of acetic acid is used to preserve fish, meat etc.

5. Methylated spirit or denatured spirit: Poisonous methanol (10%), a small amount of pyridine, naphtha, copper sulphate. etc.

6. Some polymers and their monomers:

Polymer	Name and formula of the monomer	Uses of polymer
1. Polythene or polyethene (PE)	Ethene or Ethylene (CH2 = CH2)	1. Used to prepare carry bags,2. used to prepare water pipes, water tanks, bottles, buckets, mugs etc.
2. Polyvinyl chloride(PVC)	Vinyl Chloride	1. To prepare a false ceiling, 2. to make an insulator of wire, 3. used to prepare an inner side of the refrigerator.
3. Polytetrafluroethylene (PTFE)	Tetrafluoroethylene(F2C = CF2)	1. prepare non-stick utensils,2. used as insulators in electrical goods, 3. To prepare several apparatus in the chemical lab.

 

Are mixed with ethyl alcohol to make it poisonous and non-drinkable. This mixture is termed as methylated spirit or denatured spirit.

7. Uses of denatured spirit:

1. Used as a solvent of paint or varnish.
2. Used as the fuel of spirit lamp and stove.

Physical Science Class 10 West Bengal Board

Organic Chemistry Topic C Polymer, Ethyl Alcohol, Acetic Acid And Denatured Spirit Short And Long Answer Type Questions

 

Question 1. Which compound is formed due to the polymerisation of ethylene? How does it differ from ethylene in terms of its properties?

Answer: Due to polymerisation of ethylene, the polymer polythene or polyethene is formed.

The difference in properties of ethylene and polythene are as follows—

Ethylene	Polythene
1. Ethylene is a gas at ordinary temperature.	1. Polythene is solid at ordinary temperature.
2. The molar mass of ethylene is constant. Its value is 28.	2. Polythene is formed by the combination of numerous ethylene molecules. Thus, its molar mass is very high (approx. 20000).
3. Being a gas, the molar volume of ethylene at STP is very high (22.4 L).	3. Being a solid, the molar volume of polythene at STP is very low.

 

Question 2. What are biodegradable and non-biodegradable polymers? Give examples.

Answer:

Biodegradable polymers

1. The polymers which are degraded by the enzymatic action of environmental microorganisms (such as bacteria, fungi etc.) to form simple molecules (such as CO₂, H₂O etc.) are known as biodegradable polymers.’

Example: Polymers obtained from animals and plants such as carbohydrates (cellulose, starch), protein, nucleic acids etc.

Non-biodegradable polymers

2. The polymers which are not degraded by the enzymatic action of environmental microorganisms (such as bacteria, fungi etc.) to form simple molecules such as CO₂, H₂O etc., are known as non-biodegradable polymers.

Example: Synthetic polymers such as polyethene, PVC, polystyrene, Teflon etc.

Physical Science Class 10 West Bengal Board

Question 3. Biodegradable polymers do not cause environmental pollution—Explain with a suitable example.

Answer:

Biodegradable polymers do not cause environmental pollution

Biodegradable polymers are degraded into simpler compounds such as CO₂, H₂O etc., due to the enzymatic action of different microorganisms (bacteria, fungi etc.) present in the environment. These compounds are not hazardous to the environment and thus, biodegradable polymers do not cause environmental pollution.

For example, cotton, straw, paper and wood are biopolymers or polymers originating from plant sources. Cellulose present in these biopolymers is degraded by the enzymatic action of microorganisms into compounds which are not harmful to the environment.

Question 4. How do non-biodegradable polymers cause environmental pollution?

Answer:
1. Most synthetic polymers are non-biodegradable in nature and hence, they are not easily decomposed. These polymers accumulate in the soil and prevent the free flow of air and water in the soil. This makes the soil infertile and unsuitable for agriculture.

Deposition of these polymers in the soil leads to the formation of toxic chemical compounds due to weathering which in turn adversely affects the useful soil microorganisms. Accumulation of plastics also blocks the drains and sewage canals which makes the sewage system ineffective.

2. Combustion of synthetic polymers produces poisonous gases like CO, SO₂, NO₂ etc., and causes air pollution.
3. PVC polymers may sometimes contain free vinyl chloride monomer. When water pipes, water tanks etc., made of PVC get damaged due to friction, the monometer(vinyl chloride) may mix with water, Vinyl chloride is carcinogenic in nature.

 

 

Physical Science Class 10 West Bengal Board

Question 5. What measures should be adopted to prevent pollution caused by non-biodegradable polymers?

Answer:

1. Extensive use of synthetic polymers such as plastics, polythene, PVC etc., should be reduced.
2. Recently, it has been possible to prepare some synthetic biodegradable polymers such as Nylon- 2, Nylon-6, PHBV etc. However, the cost of production of these polymers is very high.
3. Natural polymers such as paper, cotton, jute etc., are biodegradable in nature. Thus, the use of jute and paper for packaging purposes should be encouraged. The government must also initiate awareness campaigns for people to encourage the use of natural polymers.
4. Remoulding of plastic wastes into other useful substances by recycling is another way to control pollution caused by non-biodegradable polymers.

Question 6. Discuss some important uses of ethyl alcohol.

Answer:

Some important uses of ethyl alcohol

1. Ethyl alcohol is used as a solvent for resins, soaps, varnishes, rayons, scents, pigments, synthetic rubbers, synthetic fibres, medicines, etc. % 2. It is used in the preparation of ether, ethyl esters, ethyl halides, chloroform, acetic acid, ethylene, methylated spirit etc.
3. Power alcohol which is used as an automobile fuel is prepared by mixing ethyl alcohol with petrol.
4. In cold countries, a mixture of water and ethanol is used as the anti-freezing agent in the radiators of motor vehicles.

Question 7. Mention some important uses of acetic acid.

Answer:

Some important uses of acetic acid

1. Glacial acetic acid (anhydrous) is used as a solvent for many organic compounds. 0 Acetic acid is used to prepare chemical compounds such as acetone, ethyl acetate, acetic anhydride, acetyl chloride etc.
2. Cellulose acetate, used for the preparation of photographic films and synthetic fibres (rayon), is prepared from acetic acid.
3. Vinegar (5-8% aqueous solution of acetic acid) is used as a preservative for fish, meat, etc., and also for making pickles and chutneys.

Question 8. What are polymers and monomers? Give example.

Answer:

Polymers and monomers

The giant molecules of high molecular mass formed by the polymerisation of a large number of small molecules, linked together in long chains of varying lengths are called polymers. The small molecules forming the repeating units in polymers are called monomers.

Physical Science Class 10 West Bengal Board

Example: A large number of ethylene molecules combine with each other to form polyethene. Hence, polyethene is the polymer while ethylene is its monomer.

Question 9. Write the name and structural formula of the monomer of polythene. Write some lines of polythene.

Answer: The monomer of polythene is ethene or ethylene and its structural formula is CH₂ = CH₂.

Uses of polythene: It is used as a packaging material and in the manufacture of carry bags, water pipes, water tanks, bottles, buckets, and window nets and as electrical insulation for being a non-conductor of electricity.

Question 10. Write the name and structural formula of the monomer of polyvinyl chloride (PVC). Write some uses of PVC.

Answer: The monomer of polyvinyl chloride (PVC) is vinyl chloride and its structural formula is CHCI.

Uses of PVC: It is used in the manufacture of electrical wires and cable insulation, water pipes, water tanks, raincoats, artificial flooring, handbags, tubings and hoses for corrosive materials etc.

 

 

 

 

Question 11. Write the name and structural formula of the monomer of polytetrafluoroethylene (PTFE) or Teflon. Write some uses of Teflon.

Wbbse Class 10 Physical Science Solutions

Answer: The monomer of Teflon is tetrafluoroethylene and its structural formula is F₂C — CF₂.

Uses Of Teflon: It is used in making non-stick cookware, as an insulating cover in electrical appliances, in making pipes and tanks for carrying corrosive substances and for making laboratory apparatus.

Question 12. Biodegradable polymers are more eco-friendly than non-biodegradable polymers. Explain.

Answer:

Biodegradable polymers are more eco-friendly than non-biodegradable polymers.

Uncontrolled use of non-biodegradable polymers is a major cause of environmental pollution. So, the use of natural polymers such as cotton, wood, paper, jute etc., should be encouraged until and unless the cost of manufacture of biodegradable polymers can be minimised. Jute and paper should be extensively used for packaging. Jute bags instead of plastic bags should be used for carrying things. As these are biodegradable polymers, they do not cause pollution.

Wbbse Class 10 Physical Science Solutions

Question 13. Write the equation of what happens when absolute ethanol is heated with excess concentrated  H₂SO₄ at 170°C. What wifi happens if the reaction is carried out with excess ethanol instead of excess acid?

Answer: When absolute ethanol is heated with excess concentrated  H₂SO₄ at 170°C, ethanol undergoes dehydration to produce ethene.

CH₃CH₂OH + H₂SO₄ (conc.) → CH₂ = CH₂ (ethene) + H₂O
(excess)

If the reaction is carried out in presence of excess ethanol instead of excess acid at a temperature of 140°C, then diethyl ether is produced instead of ethene.

CH₃CH₂OH + HOCH₂CH₃ → CH₃CH₂ —O—CH₃CH₂ + H₂O

Question 14. Write with an equation of what happens when sodium hydroxide reacts with acetic acid.

Answer: Acetic acid reacts with sodium hydroxide solution to form sodium acetate or sodium ethanoate (CH₃COONa) and water.

CH₃COOH + NaOH → CH₃COONa + H₂O

Question 15. Write with an equation what happens when sodium bicarbonate reacts with acetic acid.

Answer: Acetic acid (CH₃COOH) reacts with sodium bicarbonate (NaHCO₃) to form sodium acetate, water and carbon dioxide and the latter comes out of the reaction mixture as effervescence.

CH₃COOH + NaHCO₃ → CH₃COONa + CO₂ ↑ + H₂O

Wbbse Class 10 Physical Science Solutions

Question 16. How will you distinguish between ethanol and acetic acid using sodium bicarbonate?

Answer: Ethanol does not react with sodium bicarbonate but, acetic acid reacts with sodium bicarbonate to form carbon dioxide which comes out of the solution as effervescence.

CH₃CH₂OH + NaHCO₃ → No reaction
CH₃COOH + NaHCO₃→ CH₃COONa + CO₂ ↑+ H₂O

Question 17. What is esterification? Describe the esterification reaction of ethyl alcohol and acetic acid along with a suitable equation.

Answer:

Esterification

1. In presence of a suitable catalyst such as concentrated H₂SO₄ or dry HCI, carboxylic acids react with dry alcohols to produce esters and water. This reaction is known as the esterification reaction.
2. When dry ethyl alcohol (CH₃CH₂OH) is heated with acetic acid (CH₃COOH) in presence of concentrated H₂SO₄ which acts as the catalyst, ethyl acetate (CH₃COOCH₂CH₃) having a fruity smell is formed.

 

 

Question 18. What are the physiological effects of consuming ethyl alcohol?

Answer:

Physiological effects of consuming ethyl alcohol

Consumption of limited amount of ethyl alcohol may act as a mild stimulant. However, if it is consumed in large quantities, it may lead to different physiological problems such as headache, nausea, reluctance to work, incoherence of speech, unconsciousness etc. Regular consumption of ethyl alcohol may lead to addiction which affects the liver and kidneys and may eventually lead to death.

Wbbse Class 10 Methanol Formula

Question 19. Briefly discuss the toxic effects of methanol.

Answer:

The toxic effects of methanol

Methanol is a highly toxic compound. Consumption of even a small amount of methanol may be fatal. It gets oxidised to form formaldehyde in the liver cells which rapidly reacts with certain components responsible for the formation of the cells. As a result, the protoplasm of the cell gets coagulated. Apart from this, methanol also damages the optic nerves which may cause blindness. Excess intake of methyl alcohol may even cause death.

Question 20. What is a methylated spirit or denatured spirit? Write its uses.

Answer:

Methylated spirit or denatured spirit

In order to prevent the use of ethanol as a beverage, it is made unfit for consumption by adding highly poisonous methyl alcohol (up to 10%) along with small amounts of certain compounds having a bitter taste such as pyridine, copper sulphate, naphtha etc. This mixture is known as methylated spirit or denatured spirit.

Uses: It is used as a solvent for paints and varnishes, as a fuel and in lighting stoves.

Wbbse Class 10 Methanol Formula

Question 21. Write down the differences between polymer and monomer.

Answer:

The differences between polymer and monomer are as follows—

Topic	Polymer	Monomer
1. Definition	Giant molecules formed by the chemical union of a large number of small molecules, Jinked together in long chains of varying lengths are called polymers.	The small molecules forming the repeating units in polymers are called monomers.
2. Molecular weight	The molecular weight of polymers is much higher than the corresponding monomers.	The molecular weight of monomers is less compared to that of polymers.
3. Formula	The formula varies with the number of repeating units and the process of polymerisation.	The formula of the monomer is fixed.
4. Example	Polytetrafluoroethylene (PTFE) or Teflon	The monomer of Teflon is tetrafluoroethylene (F2C = CF2)

 

Question 22. Which one between jute and polythene used in packaging is eco-friendly and why?

Answer: Jute is eco-friendly. Because jute is the natural polymer of carbohydrate molecule called cellulose. Being biodegradable, jute easily decomposes and mixes with soil, causing no harm to the environment. But polythene is a non- biodegradable polymer and hence does not mix with soil after use and causes environmental pollution in several ways.

Question 23. Identify A, B and C:

Answer:

 

Question 24. The molecular formula of an organic compound is   C₂H₄O₂. The compound is soluble in water and on the addition of NaHCO₃ in the aqueous solution of the compound, CO₂ gas evolved. Identify the compound. Write down the condition and balanced equation of the reaction of the organic compound with ethanol.

Answer:
1. Since the compound is organic, water-soluble and produces CO₂ in reaction with NaHCO₃, the compound must be a carboxylic acid i.e., the —  COOH group is present as a functional group.

Since the formula of the compound is C₂H₄O₂ so — CH₃ group must be added with — COOH.

The compound is CH₃COOH i.e., acetic acid.

Wbbse Class 10 Methanol Formula

2. In presence of heat and cone. H2S04, acetic acid reacts with ethyl alcohol producing sweet-smelling ethyl acetate ester and water.

 

Organic Chemistry Topic C Polymer, Ethyl Alcohol, Acetic Acid And Denatured Spirit Answer In Brief

 

Question 1. Which polymer is present in plant fibres such as cotton, jute etc.?
Answer: Plant fibres such as cotton, jute etc., are made of the polymer, cellulose.

Question 2. What is bio pool?
Answer:

Bio pool

The trade name for polyhydroxy butyrate (PHB) is bio pool. It is an eco-friendly and biodegradable synthetic polymer.

Question 3. Give an example of a biodegradable synthetic polymer.
Answer: Polyhydroxybutyrate is a biodegradable synthetic polymer.

Question 4. What are the uses of bio pool?
Answer: Biopol is widely used to make single-use products such as disposable cups, shaving razors, surgical threads etc.

Question 5. What is rectified spirit?
Answer:

Rectified spirit

A solution of 95.6% ethanol and 4.4% water is known as rectified spirit.

Question 6. Give an example of an organic compound which turns blue litmus red.
Answer: Acetic acid turns blue litmus red.

Question 7. Which organic compound is used to make pickles and chutneys?
Answer: Vinegar (5-8% aqueous solution of acetic acid) is used to make pickles and chutneys.

Wbbse Class 10 Physical Properties Of Organic Compounds

Question 8. What is vinegar?
Answer:

Vinegar

A 5-8% aqueous solution of acetic acid is commonly known as vinegar.

Wbbse Class 10 Methanol Formula

Question 9. How do biopolymers decompose in the natural environment?
Answer: Biopolymers decompose into simple molecules (like  CO₂, H₂O etc.) by the action of different microorganisms (fungi, bacteria etc.) present in the natural environment.

Question 10. Write down the name of the monomer of polythene.
Answer: The monomer of polythene is ethene or ethylene.

Question 11. What is PVC?
Answer:

PVC

PVC or polyvinyl chloride is the polymer of vinyl chloride (CH₂ = CH — Cl).

Question 12. Mention the use of polyvinyl chloride.
Answer: PVC is used to prepare corrugated roofing material.

Question 13. Name a polymer which is used to prepare raincoats, sandals or gumboots.
Answer: PVC or polyvinyl chloride.

Question 14. Write down the full form of PTFE.
Answer: Polytetrafluoroethylene.

Question 15. Mention the use of polytetrafluoroethylene.
Answer: Polytetrafluoro ethylene is used to prepare non-stick cooking utensils.

Question 16. Name two natural polymers.
Answer: Cellulose and protein.

Question 17. Which type of polymer is protein?
Answer: Protein is a biodegradable natural polymer.

Question 18. Name the monomer of the protein.
Answer: The monomer of protein is an amino acid.

Question 19. Mention an use of ethyl alcohol.
Answer: Ethyl alcohol is used to prepare rectified spirit (96.5% ethanol and 4.4% H₂O) which is used as an antiseptic.

Question 20. What is formed when ethyl alcohol is dehydrated by cone H₂SO₄?
Answer: Ethylene is formed when ethyl alcohol is dehydrated by the cone. H₂SO₄ .

Question 21. Write down the formula of a compound which can form ester in reaction with ethanol.
Answer: Acetic acid (CH₃COOH)

Question 22. Which gas is evolved when NaHCO₃ is added to acetic acid?
Answer: Carbon dioxide (CO₂)

Question 23. Which compounds are formed in the reaction of CH₃COOH with NaOH?
Answer: Sodium acetate (CH₃COONa) and water (H₂O).

Question 24. What is glacial acetic acid?
Answer:

Glacial acetic acid

Acetic acid that contains a very low amount of water (less than 1%) is called anhydrous acetic acid or glacial acetic acid. The reason it is called glacial is that it solidifies into white solid acetic acid crystals at 16.7°C.

 

Organic Chemistry Topic C Polymer, Ethyl Alcohol, Acetic Acid And Denatured Spirit  Fill In The Blanks

 

Question 1. Ethyl alcohol reacts with metallic sodium at ______ temperature to liberate _______ gas.
Answer: Ordinary, hydrogen

Question 2. The monomer of Teflon is _____
Answer: Tetrafluoroethylene

Question 3. ______ is mixed with petrol to produce power alcohol which is used as an automobile fuel.
Answer: Ethanol

Question 4. Ethyl alcohol on dehydration produces _____
Answer: Ethylene

Question 5. A mixture of 80% _____ and 20% ______ is used as an anaesthetic during surgeries.
Answer: Ethylene, oxygen

Question 6. Among all alcohols, _________ is toxic in nature.
Answer: Methanol

Question 7. Iodine dissolved in _______ is known as a tincture of iodine.
Answer: Ethyl alcohol

Question 8. The monomer of PVC is _______
Answer: Vinyl chloride

Question 9. ______ is used to prepare gramophone records.
Answer: PVC

Question 10. The polymer of phenol and formaldehyde is _______
Answer: Bakelite

Question 11. Ethyl alcohol reacts with metallic sodium at _______ temperature to form gas.
Answer: Normal, hydrogen

Question 12. _____ is mixed with petrol to form the fuel for a motor car called power alcohol.
Answer: Ethanol

Question 13. Almost ______ ethyl alcohol is present in rectified spirit.
Answer: 95.6%

Question 14. _______ can cause blindness by causing harm to the optic nerve.
Answer: Methanol

 

Organic Chemistry Topic C Polymer, Ethyl Alcohol, Acetic Acid And Denatured Spirit State Whether True Or False

 

Question 1. Oxygen or peroxide is used as the catalyst during the polymerisation of ethylene.
Answer: True

Question 2. The monomer of the polymer Teflon is vinyl chloride.
Answer: False

Question 3. Methanol on entering the body can damage the optic nerves.
Answer: True

Question 4. Polystyrene and polyethene are examples of non-biodegradable polymers.
Answer: True

Question 5. Petrol mixed with alcohol along with a cosolvent like benzene is called power alcohol.
Answer: True

Question 6. The basicity of acetic acid is 2.
Answer: False

Question 7. In spite of being an organic compound, ethyl alcohol is soluble in water due to its formation of hydrogen bonds with water molecules.
Answer: True

Question 8. The reaction between an alcohol and an aldehyde to form an ester in presence of concentrated sulphuric acid is called an esterification reaction.
Answer: False

Question 9. Teflon is used to prepare non-stick frying pans.
Answer: True

Question 10. Ethanol causes harm to the liver.
Answer: True

Question 11. Denatured spirit is used as a solvent of organic substances.
Answer: True

Question 12. Methylated spirit is poisonous due to the presence of methanol.
Answer: True

Miscellaneous Type Questions Match The Column

Question 1.

Column A	Column B
Dimethyl ether and ethyl alcohol	1. Chain isomers
n-propyl alcohol and propan-2-ol	2. Functional group isomers
n-pentane, isopentane and neopentane	3. Ring-chain isomers
Propene and cyclopropane	4. Positional isomers

 

Answer:
Dimethyl ether and ethyl alcohol: 2. Functional group isomers
n-propyl alcohol and propan-2-ol: 4. Positional isomers
n-pentane, isopentane and neopentane: 1. Chain isomers
Propene and cyclopropane: 3. Ring-chain isomers

Question 2.

Column A	Column B
Hydrogenation of ethylene	1. Undergoes substitution reactions
C4H10	2. Prepared acetic acid and methane from their constituent elements
Kolbe and Berthelot	3.  Alternative name for alkanes
Paraffin	4.  Ethane is formed as the product

 

Answer:
Hydrogenation of ethylene: 4.  Ethane is formed as the product
C₄H₁₀ :  1. Undergoes substitution reactions
Kolbe and Berthelot: 2. Prepared acetic acid and methane from their constituent elements
Paraffin: 3.  Alternative name for alkanes

Question 3.

Column A	Column B
Biopol	1. Natural polymer
Vinegar	2. Commercial name of tetrafluoroethylene
Teflon	3. Commercial name of polyhydroxybutyrate
Cellulose	4. 5-8% aqueous solution of acetic acid

 

Answer:
Biopol: 3. Commercial name of polyhydroxybutyrate
Vinegar: 4. 5-8% aqueous solution of acetic acid
Teflon: 2. Commercial name of tetrafluoroethylene
Cellulose: 1. Natural polymer

WBBSE Solutions for Class 10 Physical Science and Environment