Chapter 1 Physical Environment Long Answer Type Question
Question 1. What are the differences between a real image and a virtual image?
Answer:
Differences Between a real image and a virtual image
| Real image | Virtual image | ||
| 1 | Real images are formed when reflected rays or refracted rays actually meet at the same points. | 1 | Virtual images are formed when the reflected or the refracted rays appear to diverge from other points. |
| 2 | It can be cast on a screen. | 2 | It cannot be cast on a screen. |
| 3 | By single reflection or refraction, it is always inverted. | 3 | It is always erect. |
| 4 | It may be magnified or diminished or equal to the size of an object. | 4 | Images formed by a plane mirror are equal to the size of the object.
Those formed by lenses or curved mirrors may be equal, larger or smaller in size in comparison to the object. |
Question 2. Describe the formation of a real image with the help of a magnifying glass. What is the angle, in degrees, between a plane mirror and the straight line joining the object and its?
Answer:
Formation Of A Real Image With The Help Of A Magnifying Glass:-
Let us take a magnifying glass and hold it in the sunlight above a piece of white paper, placed on the floor. We can see a round shape of light on the paper.
It is nothing but the image of the sun, which can be cast on the paper, which is the “screen” here and the image is a real image of the sun because,
after refraction, the rays of light coming from the sun actually meet at some point. The formation of a real image of magnifying glass has been The angle is 90°.
WBBSE Class 8 Light long answer questions
Question 3. What are the characteristics of images formed by a plane mirror? If a diverging beam of light is incident on a plane mirror, is it reflected as a converging, parallel, or diverging beam?
Answer:
Characteristics Of Images Formed By A Plane Mirror
Formation of multiple images by plane mirrors
Let us now take two plane mirrors and place them vertically; on a white sheet of paper in such a way that the angle between them is 90°,
If we now place an object (say an eraser) between the mirrors, we can see multiple images. In fact, we can see three images when the angle between the two mirrors is 90°.
If the angle between the mirrors is 45°, then the number of images formed is 7. So, to generalize our observation,
we can say that if the angle between the two plane mirrors is x°, then the number of images formed will be equal to (360°/x°-1), if 360°/x° is an even integer and (360°/x°),360°/x° is an odd integer
Read And Learn More WBBSE Solutions For Class 8 School Science Long Answer Type Questions
So, when two plane mirrors are placed vertically facing each other and if an object is placed between them then a large number of images are formed.
Diverging beam
Long answer type questions on light for Class 8
Question 4. Describe the construction of the periscope. Mention one disadvantage of periscope.
Answer:
Construction Of The Periscope:-
It is a simple long, tubular instrument with which a viewer can see different objects from the other side of a barrier that extends high above his or her head and are out of the direct line of sight.
It consists of a long rectangular box made of wood or metal. Two plane mirrors M1 and M2 (or in some cases two prisms) are fixed inside the box, one at the top and the other at the lower end of the box such that the mirrors face each other.
Each mirror makes a 45° angle with the axis of the periscope box. Rays of light coming from a distant object are incident on the mirror Mr. The rays get reflected by M1 and are incident on mirror M2.
The mirror M2 then reflects the reflected rays of light towards the eyes of the observer. The observer thus sees any object from the other side of a high barrier.
Soldiers use it to observe the movements of enemies keeping themselves hidden in trenches. In submarines, a periscope is used to watch the movements of the enemy vessel on the surface of the water, while remaining submerged in the water.
Sports lovers, unable to get entry into the galleries of a playground, take the help of a periscope to watch games from outside the barriers of the playground.
The disadvantage of periscope: The final image is not bright due to successive reflections in a periscope.
Question 5. What are the differences between simple reflection and total internal reflection?
Answer:
Differences Between Simple Reflection And Total Internal Reflection
| Simple Reflection | Total Internal Reflection | ||
| 1 | Simple reflection occurs when light from any medium is incident on a reflector. | 1 | In the case of total internal reflection, the ray of light must pass from an optically denser medium to an optically rarer medium. |
| 2 | Reflection occurs for any value of the angle of incidence. | 2 | In this case, the angle of incidence in the optically denser medium must be greater than the characteristic critical angle of the two media. |
| 3 | In the case of simple reflection, a part of the light is absorbed by the reflector, another part is refracted and the remaining is reflected. | 3 | In this case, the incident light totally reflects back from the surface of separation of the two media and no part of the incident light is either absorbed by the medium or refracted. |
| 4 | For simple reflection, a reflector is required. | 4 | In the case of total internal reflection, the surface of separation between the two media itself acts as a reflector. |
WBBSE Chapter 1 light detailed answers
Question 6. Explain with a suitable the formation of mirages in the desert during day time.
Answer:
Total Internal Reflection
When a ray of light tends to travel from an optically denser medium to an optically rarer medium, then if the angle of incidence (∠DON1 exceeds the critical angle (°C), the incident light totally reflects back along OR to the first medium (optically denser medium)
Refraction of light does not take place in this case. Such a phenomenon is called total internal reflection.
The term “total” is used because of the incident media. The points at which total internal light totally reflects back into the same denser reflection takes place look very bright,
as the medium from the surface of separation of the two incident lights reflects totally from these points.
The conditions required for the total internal reflection to take place are :
- light rays should travel from the denser to the rarer medium.
- The angle of incidence should be greater than the critical angle for the pair of media involved
- Phenomena related to total internal reflection
1. Brightness of diamonds or gems
Usually, diamonds and other gems are constituted of materials of high refractive index, the critical angle of each of which with respect to air is thus very small.
For example, the critical angle of a diamond is only 24.5°. Also, diamonds or any other gem is cut in such a way that, light can get into it through all surfaces but can emerge from very few surfaces.
This is because, the diamond or the gemstone is cut in such a way, that the rays within the “body” trying to come out are incident on most of the surfaces at an angle exceeding the critical angle.
After undergoing several total internal reflections, the light rays are incident on a small number of surfaces at angles less than the critical angle and emerge from those surfaces only.
Hence, the emergent light is very intense, and that is why a diamond or a gem looks very bright.
2. A crack in the glass of a window pane looks shiny
Some air is present in the gap of a crack in the glass. So, light rays pass through the denser medium (i.e. glass)
when tend to pass through the rarer medium (i.e. air), and total internal reflection occurs at some point in the crack. Hence those points of the crack look shiny.
3. An empty test tube dipped in the water looks shiny
An empty test tube is dipped in water in an inclined way. Light rays passing through water outside the tube tend to pass through air present in the empty test tube.
Thus light passes from an optically denser medium to an optically rarer medium. At some points on the surface of the test tube, rays of light are incident at angles exceeding the critical angle of water to air.
At those points total internal reflection takes place and so the empty portion of the test tube looks bright when viewed from above vertically.
4. Drop Of Water On the Arum Leaf Seems Glittering
This is because when a ray of light travels from inside the water droplet to the air, the angle of incidence exceeds the critical angle of the two media (i.e. water and air).
So, total internal reflection occurs at the surface of the separation between water and air. When the emerging ray of light reaches to viewer’s eye, the viewer finds the area glittering.
5. Mirage in the desert
In deserts, during day time, the sand bed becomes extremely hot. So the air just above it is also heated and the density of air decreases.
With increasing altitude, the successive layers of the air have gradually increasing density. In absence of any flow of air, this is maintained for a long time.
Let us consider a light ray coming from point “A” on top of a tree in the desert moving downwards
As the density of air decreases downwards, and as the ray of light moves downwards through different layers (of decreasing density), the angle of refraction increases progressively.
At some interface between two layers of air, the angle of incidence is greater than the critical angle, and it suffers total internal reflection and consequently moves upwards.
As it moves upwards from a rarer medium to a denser medium, the ray of light bends towards the normal. When the ray reaches an observer, he or she “secs” a virtual image of A at A7.
In this way, rays coming from different parts of the object (i.e. tree) reach to viewer’s eye after suffering total internal reflection.
Ultimately the viewer sees an inverted, virtual image of the original object in a direction far away from the original position of the object.
Due to variations in temperature, the density of different layers of air changes continuously, and to an observer, the image seems shimmering.
The observer thinks this inverted, shimmering image of the tree is the reflection of the tree formed on the water surface below the tree and the viewer is totally misguided. This optical illusion is called a mirage.
6. Mirage in the cold country
In colder countries, the air in contact with water is denser and with increasing altitude, the density of air decreases.
For our convenience, we can think of different layers of air of decreasing density with increasing altitude.
The rays of light from a boat far away from the jetty, when going in an upward direction, it travels from a denser to a rarer medium.
In each layer, the refracted ray moves away progressively from the normal and the angle of incidence increases gradually.
Ultimately, a point is reached when the angle of incidence becomes greater than the critical angle of the two adjacent layers of air and the incident ray suffers total internal reflection at that particular interface
(or surface of separation) and bends downwards. When it reaches the viewer’s eye, the viewer sees a virtual image of the boat, which is inverted, moving in the sky.
In-depth explanations of light concepts for Class 8
Question 7. What do you mean by looming?
Answer:
Looming:-
The velocity of light in a vacuum is 3×1010 cm/s. The refractive index of glass with respect to air is 1.5. What is the velocity of light in a glass medium?
Looming is an optical phenomenon observed in cold countries due to total internal reflection, wherein objects like ships, which are normally below the horizon appear to be hanging in air.
The refractive index of a glass
= velocity of light in vacuum/velocity of light in the glass
or, 1.5= 3×1010 / velocity of light in the glass
or, 1-5 – velocity of light in the glass
or, the velocity of light in the glass
= (3x 1010)/1.5 = 2 x 1010 cm/s.
Question 8. What is the value of the absolute critical angle of the denser medium as shown in the adjacent ?
Answer:
Mention the conditions necessary for total internal reflections to occur.
Answer: Absolute critical angle, 0c = 48°.
Conditions for total internal reflections:
Question 9. When a light ray travels from a rarer to a denser medium, how does the bending of light depend on the refractive index of the material? Why does refraction occur?
What is the angle of incidence if a ray is reflected by a plane mirror back along its original path?
Answer:
The higher the refractive index of the material, the more the light bends as it is refracted. Cause of refraction: This is the case of normal incidence wherein, Zi = Zr = 0°.
Refraction of Light
It is our common experience that when light travels from one medium to the other, it deviates from its original path.
If we dip a pencil obliquely in a beaker containing water and observe it from a particular position as shown in it seems that the pencil is bent at the point of contact between the water surface and air.
(It is also called the air-water interface). This occurs due to the optical phenomenon known as refraction.
It means that light deviates from its path if one optical medium is changed with another optical medium or the density of the same medium changes because of variations in temperature, pressure, etc.
The path of light remains a straight line path in the second medium but it is inclined at some angle with the original path in the first medium.
The phenomenon, due to which a ray of light deviates from its original path while traveling from one optical medium to another optical medium is called refraction.
1. Refraction of light in glass slab
Let us discuss the phenomenon in some detail. Let an oblique ray of light traveling through air transmit through a glass slab and then come out of the glass slab to air again The ray AO in the first medium is called the incident ray.
If no deviation of path occurs at 0 then the light would have traveled along OA’. But the path of the light deviates at O and light moves through the glass medium along OB.
OB is the refracted ray. Again, at B there is a glass-air interface, and the deviation of the path of the light is observed and in the air medium it is refracted along BC.
BC is called the emergent ray. The angle between the incident ray and normal (i.e. NIST) is the angle of incidence (i.e. ∠AON) and the angle between the refracted ray and the normal at the
2. Finding the refractive index of glass with respect to air
Let us take a white sheet of paper and place it on the surface of a table. Let us now place a glass slab and place it at the middle of the paper and draw its boundary ABCD with a pencil.
Now place two board pins P and Q in an upright position towards the AB side of the glass slab, as
Now looking from the side CD, let us fix two more board pins R and S such that these two pins and the images of pins P and Q as seen through the glass slab are in the same straight line.
The glass slab and the pins are then removed and the position of the pins is marked. Join PQ to meet AB at point O and join SR to meet CD at L.
Draw NM such that it is perpendicular to AB at point
Now with O as the center, let us draw a circle of any radius, intersecting PO at E and OL at G. Draw EF and GH in such a way that EF is perpendicular to NO and GH is perpendicular to MO.
Understanding light concepts long answers for Class 8
So PQ. is the incident ray, 0 is the point of incidence, OL is the refracted ray, ∠EON is the angle of incidence and ∠LOM is the angle of refraction.
The ratio of EF and GH is determined. Now if the angle of incidence is changed, the angle of refraction will also change. But in each case the ratio, EF/GH will remain constant.
[More appropriately it can be said that if the two media are fixed and the color of the light remains unchanged during refraction, then whatever the angle of incidence, the magnitude of EF/GH remains constant.]
The ratio is called the Refractive Index of the second medium (i.e. here it is glass) with respect to the first medium (i.e. here it is air).
When refraction occurs between a vacuum and a certain medium, then the refractive index of the said medium is called Absolute Refractive Index.
The refractive index depends on the nature of the two media and on the color of a ray of light. When a ray of light travels from an optically denser medium to an optically rarer medium, then the magnitude of the refractive index for the light of different colors will be in the order: of red< green < blue < violet.
When a ray of light travels from an optically rarer medium, such as air, to an optically denser medium (say glass), the ray in the denser medium moves towards the normal and the angle of incidence is always greater than the angle of refraction (i.e. Z/ > Zr).
medium, such as glass, to an optically rarer medium (say air), then the ray of light in the rarer medium moves away from the normal and the angle of incidence is always less than the angle of refraction (i.e. Z/ < Zr).
It should be kept in mind that the density of a medium and its optical density is not the same. For example, oil is optically denser than water, but its natural density (i.e. mass per unit volume) is less than that of water.
Velocity Of Light In Rarer And Denser Medium
A medium in which light moves faster or the velocity of light is higher is optically rarer whereas a medium in which light moves slower is optically denser.
Whether a medium is optically denser or rarer depends upon its absolute refractive index. Light has a constant velocity of 3 x 108 m/s for all colors in a vacuum (or air).
However, if the light travels through any other optical medium, it is slowed down. It is this slowing down of light that is responsible for the phenomenon of refraction.
It has been found experimentally that, Absolute refractive index of a medium Velocity of light in a vacuum “ /Velocity of light in that medium
Laws of Refraction
First Law: The incident ray, the refracted ray, and the normal drawn at the point of incidence on the refracting surface lie on the same plane.
Second Law: For refraction of an obliquely incident ray of light of any given color in a given pair of optical media, the refractive index is constant, irrespective of the magnitudes of the angle of incidence and angle of refraction.
In the beginning, we mentioned that when water is poured into an empty tub, the base of the tub appears to be raised.
This occurs due to refraction. Now we can explain this in a better way. Let us consider a point “0” at the base of the tub.
When water is poured into the tub, the light from “0” travels from water (an optically denser medium) to air (an optically rarer medium).
When the ray of light enters in air, it moves away from the normal drawn surface of separation between the two media.
If the refracted rays are extrapolated linearly backward, they meet at point “O'”, which is positioned higher than “0” So, to a viewer, it seems that the base of the tub has been raised
Key long answer questions about light for Class 8
4. Critical Angle
If a ray of light starting from an optically denser medium refracts in a rarer medium, for all oblique incident rays, the angle of refraction is greater than the angle of incidence.
In the adjacent AO is an obliquely incident ray. Its corresponding refracted ray is OA’ and the angle of refraction is ∠NOA’ which is greater than the corresponding angle of incidence, ∠AON’.
If the angle of incidence is gradually increased, the corresponding angle of refraction is also gradually increased.
This continues till for a certain angle of incidence, ∠CON’ (denoted as °C) the corresponding angle of refraction, ∠GON, becomes 90°
That means the refracted ray, OG, grazes along the surface of the separation of the two media. This particular angle of incidence for which the angle of refraction becomes 90° is called the critical angle.
Its value depends on the pair of media as well as the color of the incident light.
Definition: When a ray of light of any given color tends to travel from an optically denser medium to an optically rarer medium, then for a certain angle of incidence the angle of refraction is 90°.
The corresponding angle of incidence is the critical angle of the given pair of media for the given color of light.
It is to be noted that the critical angle for a certain color of light is different in different pairs of media.
The critical angle of glass with respect to air for yellow light is 42°, but that of water with respect to air is 49°. Also, a given pair of media have different critical angles for different colors of light.
WBBSE Class 8 Science practice long answer questions on light
Question 10. How many images will be formed when an object is placed between two parallel plane mirrors with their reflecting surfaces facing each other? Why do more distant images appear fainter?
Answer:
Two plane mirrors are mounted at right angles to each other. Let an object be placed in front of them. How many images are formed in all? Draw the ray to illustrate the situation.
Kaleidoscope
This is a kind of funny toy that utilizes the property of formation of multiple images of an object when placed between three or more mirrors.
Suppose, three rectangular pieces of mirrors of the same size are joined together to give it a prism-like appearance
It is joined in such a way that the reflecting surface of each mirror should face inside.
Then one of the open ends is covered by a ground glass of the required size. Some broken, colored glass pieces or colored small objects like beads, etc.
They are placed within it. If this instrument is now aimed at a suitable source of light and is seen from the other open end, we can see some beautiful patterns formed due to multiple image formations by the three mirrors.
If we spin this kaleidoscope slowly, numerous colorful patterns will be created continuously. Formation of images by two mirrors: Ray.
