WBBSE Solutions For Class 10 Physical Science And Environment Chapter 5 Light

Light Topic A Reflection Of light At Spherical Mirror Synopsis:

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.

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:

 

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]

 

 

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.

 

 

 

 

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.

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.

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. Absolute refractive index of a medium with increase of temperature.
Answer:

Question 7. When light travels from a rarer to a denser medium, its speed _____
Answer: Decreases

Question 8. 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°

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°

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

 

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. I 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 X 10¹⁴ Hz
2. 1.6 x 10¹⁴ Hz
3. 2.3 x 10¹⁴ Hz
4. 1.5 x 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

Wb 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