Living Science 2020 2021 Solutions for Class 8 Science Chapter 17 Refraction And Dispersion Of Light are provided here with simple step-by-step explanations. These solutions for Refraction And Dispersion Of Light are extremely popular among class 8 students for Science Refraction And Dispersion Of Light Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Living Science 2020 2021 Book of class 8 Science Chapter 17 are provided here for you for free. You will also love the ad-free experience on Meritnation’s Living Science 2020 2021 Solutions. All Living Science 2020 2021 Solutions for class 8 Science are prepared by experts and are 100% accurate.
Page No 225:
Question 1:
When light enters water from air, what change is observed in its speed?
Answer:
Water is optically denser medium as compared to air, so the speed of the light decreases when it enters the water from the air.
Page No 225:
Question 2:
The change in speed (Q. 1) causes a change in the direction of the light ray. What change is observed when light goes from: a. air to water b. water to air?
Answer:
(a) When a light ray travels from air into water, it's speed decreases. So, it bends towards the normal to the interface between the water and the air.
(b) When a light ray travels from water into the air, it's speed increases. So, it bends away from the normal to the interface between the water and the air.
Page No 225:
Question 3:
What is the relation between the incident ray on a glass slab and the emergent ray?
Answer:
Emergent ray from a glass slab is always parallel to the incident ray but with a lateral shift in position.
Page No 225:
Question 4:
Which is deviated more by a glass prism- red light or violet light?
Answer:
Violet color has the least wavelength and red color has the maximum wavelength. So, violet colored light deviates the most and the red colored light deviates the least among the seven component colors of the white light.
Page No 229:
Question 1:
What type of lens is thicker in the middle than at the edges?
Answer:
A converging or convex lens is thicker in the middle than at the edges.
Page No 229:
Question 2:
What effect does a convex lens have on parallel rays of light incident on it? In what way is the effect of a concave lens different?
Answer:
When a parallel beam of light is incident on a convex lens, it converges all the rays at its focus. Whereas when a parallel beam of light is incident on a concave lens, it diverges all the rays away from its focus.
Page No 229:
Question 3:
A ray of light passing through a certain point of a lens does not undergo any deviation. What point is this?
Answer:
That point is known as the 'Pole' of the lens. When a ray of light passes through the pole of a lens, it does not deviate from its path.
Page No 229:
Question 4:
What kind of lens always forms a virtual image of an object?
Answer:
A diverging or concave lens always forms a virtual image of an object.
Page No 235:
Question 1:
The eye can control the amount of light falling on the retina. Which part of the eye is responsible for this?
Answer:
The part of the eye which is responsible for controlling the amount of light entering into the eye is known as 'Iris'.
Page No 235:
Question 2:
The eye can change the focal length of its lens. Which part of the eye is responsible for this? How does the changing focal length help us?
Answer:
Ciliary muscles are responsible for changing the focal length of the eye lens when needed. Changing the focal length of the eye lens helps us see a clear image of the objects at various distances and at different viewing angles.
Page No 235:
Question 3:
A person cannot see near objects clearly. What kind of lens should his spectacles have to correct this- convex or concave?
Answer:
A person who can not see the nearby object clearly must be suffering from Long-sightedness or hypermetropia. To correct this defect of vision, his spectacles should have a convex lens.
Page No 235:
Question 4:
What is the farthest distance you can see if you have a normal eye?
Answer:
The farthest distance up to which a normal eye can see is infinity.
Page No 235:
Question 5:
Braille is a language different from English and other languages. Do you agree? Give reasons.
Answer:
Braille is a language for blind persons. It's not just a language but moreover a different way of reading for blind people. Those who can not see are taught this different language so that they can read and understand things too.
Page No 237:
Question 1:
If medium A is optically denser than medium B, then the speed of light is
(a) the same in both mediums.
(b) higher in medium A than in medium B.
(c) higher in medium B than in medium A.
(d) higher in medium A or B depending on which medium is thicker.
Answer:
(c) higher in medium B than in medium A
The speed of light decreases in a denser medium.
Page No 237:
Question 2:
When light travels from an optically denser medium to a rarer medium.
(a) it bends away from the normal.
(b) it bends towards the normal.
(c) it is undeviated.
(d) it is laterally displaced.
Answer:
(a) it bends away from the normal
When light travels from an optically denser to a rarer medium, it bends away from the normal.
Page No 237:
Question 3:
The splitting of white light into its constituent colours is called
(a) refraction.
(b) dispersion.
(c) deviation.
(d) displacement.
Answer:
(b) dispersion
The splitting of white light into its constituent colours is called dispersion.
Page No 237:
Question 4:
At what position of the object does a convex lens act as a magnifying glass?
(a) between F and 2F
(b) between F and O
(c) beyond F
(d) beyond 2F
Answer:
(b) between F and O
In this case, the image formed is virtual, erect and bigger than the object.
Page No 237:
Question 5:
Which of the following is true for a person suffering from myopia?
(a) can see far-off objects clearly but near objects appear blurred
(b) can be corrected using a convex lens
(c) far point is at finite distance and not at infinity
(d) near point is beyond 25 cm
Answer:
(c) far point is at finite distance and not at infinity
In myopia or shortsightedness, the far point is not at infinity but at some finite distance.
Page No 237:
Question 6:
If a person is suffering from hypermetropia, which object is he/she most likely to see blurred?
(a) object 25 cm away
(b) object 10 m away
(c) object 100 m away
(d) object at infinity
Answer:
(a) object 25 cm away
In hypermetropia or longsightedness, a person is unable to see nearby objects clearly. A distance of 10 cm cannot be the answer, as at this distance even the vision of a person with normal eyes will also get blurred. Therefore, the object placed at a distance of 25 cm from the eye will look blurry to a person suffering from hypermetropia.
Page No 237:
Question 7:
Which one of these controls the amount of light entering the eye?
(a) pupil
(b) iris
(c) cornea
(d) ciliary muscles
Answer:
(b) iris
Iris controls the amount of light entering our eyes.
Page No 237:
Question 8:
Myopia is corrected by using spectacles with
(a) convex lenses.
(b) concave lenses.
(c) glass slabs.
(d) prisms.
Answer:
(b) concave lenses
Concave lenses diverge the parallel rays from a distant object in such a way that it starts appearing closer to the defective eyes.
Page No 237:
Question 9:
When white light passes through a prism, it is
(a) deviated and dispersed.
(b) deviated but not dispersed.
(c) laterally displaced and not dispersed.
(d) reflected.
Answer:
(a) deviated and dispersed
When white light passes through a prism, it gets deviated and dispersed.
Page No 237:
Question 10:
How many dots does the Braille system use?
(a) 5
(b) 6
(c) 10
(d) 11
Answer:
(b) 6
Braille system uses 6-dot cell.
Page No 237:
Question 1:
Refractive index can be defined for both transparent and opaque objects. True or false?
Answer:
False.
Since, opaque objects don't allow light to pass through them, they will also have no refractive index.
Page No 237:
Question 2:
Light bends as it passes from one medium to another. What is this phenomenon called?
Answer:
The phenomenon in which light bends away from its actual path while passing from one medium to the other is called refraction.
Page No 237:
Question 3:
If light enters from water to air it bends __________ the normal.
Answer:
When light passes from water to air, it bends away from the normal.
Page No 238:
Question 4:
A pencil dipped obliquely in water appears bent at the surface due to dispersion. True or false?
Answer:
False.
The pencil, obliquely dipped in water, will appear bent at the surface due to refraction and not dispersion.
Page No 238:
Question 5:
In a prism, the emergent ray is parallel to the incident ray. True or false?
Answer:
False.
In a prism, the emergent ray is not parallel to the incident ray. The emergent ray deviates from the direction of incidence by an angle of deviation.
Page No 238:
Question 6:
A transparent medium bends light by the same amount irrespective of the colour of light. True or false?
Answer:
False.
A transparent medium bends light of different colours by different amount.
Page No 238:
Question 7:
Which of these has a smaller focal length−a thick lens or a thin lens?
Answer:
A thick lens has a smaller focal length, as it bends light more than a thin lens.
Page No 238:
Question 8:
If an object is situated at the focal point of a convex lens, where is the image formed?
Answer:
If an object is situated at the focal point of a convex lens, the image will be formed at infinity.
Page No 238:
Question 9:
A diverging lens forms a real image of an object, placed beyond F, and a virtual image of an object placed between O and F. Ture or false?
Answer:
False.
A diverging lens will always form a virtual image, as after refraction, the rays coming out of the lens will also diverge.
Page No 238:
Question 10:
The far point of a hypermetropic eye is at a finite distance from the eye. True or false?
Answer:
False.
Hypermetropic eyes can see distant objects clearly. This means the far point of a hypermetropic eye is at infinity.
Page No 238:
Question 11:
Which lens is used to correct myopia−convex or concave?
Answer:
Concave lens diverges the parallel rays coming from a distant object; therefore, it is used to correct myopia.
Page No 238:
Question 12:
A normal eye cannot clearly see objects closer than __________ cm.
Answer:
A normal eye cannot see objects closer than 25 cm.
Page No 238:
Question 13:
In cataract, the lens of the eye becomes cloudy. True or false?
Answer:
True. Cataract is the condition in which the lens in our eye becomes cloudy.
Page No 238:
Question 14:
Write Braille codes for letters A and Y.
Answer:
Letter Braille code
A
Y
Page No 238:
Question 1:
What do you mean by refraction of light? In which direction will light bend when travelling from an optically rarer medium to an optically denser medium?
Answer:
The phenomenon in which light bends while passing from one medium to another is called refraction.
Light bends towards the normal when travelling from an optically rarer to an optically denser medium.
Page No 238:
Question 2:
Why does refraction occur?
Answer:
The speed of light changes when it passes from one medium to the other with different optical density. This causes the refraction of light.
Page No 238:
Question 3:
What is dispersion?
Answer:
The splitting up of light into its constituent colours while passing from one medium to the other is called dispersion.
Page No 238:
Question 4:
Which medium will bend light more−glass or water? Why?
Answer:
Since glass is optically denser than water, it will bend light more than water.
Page No 238:
Question 5:
What is 'white light'?
Answer:
The mixture of seven colours of light namely violet, indigo, blue, green, yellow, orange and red is known as white light.
Page No 238:
Question 6:
What is the order of colours in a spectrum of 'white light'?
Answer:
In a spectrum of white light, the order of the colours is red, orange, yellow, green, blue, indigo and violet.
Page No 238:
Question 7:
Distinguish between near point and far point of the eye.
Answer:
| Near point of the eye | Far point of the eye |
| The least distance at which an eye can see an object clearly is called the near point of the eye. | The maximum distance at which an eye can see an object clearly is called the far point of the eye. |
Page No 238:
Question 8:
What is the 'blind spot' of the eye?
Answer:
The junction of the optic nerve and retina is devoid of sensory cells; hence, any image formed here cannot be seen. This is called the blind spot.
Page No 238:
Question 1:
What is the difference between the displacement produced by a rectangular glass slab in a ray of light and the deviation produced by a glass prism?
Answer:
In case of displacement produced by a rectangular glass slab, the emergent ray remains in the same direction as the incident ray. However, in case of deviation produced by a glass prism, the emergent ray deviates from the direction of the incident ray by an angle of deviation.
Page No 238:
Question 2:
Explain the term 'spectrum'. Why does a glass split white light into its constituent colours?
Answer:
The term spectrum refers to the order and arrangement of different colours of light constituting the white light.
A glass splits white light into its constituent colours because different colours of light have different angles of deviation.
Page No 238:
Question 3:
Distinguish between converging and diverging lenses.
Answer:
| S. No. | Converging lens | Diverging lens |
| 1. | A converging (convex) lens is thicker in the middle and thinner at the edges. | A diverging (concave) lens is thinner in the middle and thicker at the edges. |
| 2. | A converging lens bends a ray of light inwards. | A diverging lens bends a ray of light outwards. |
| 3. | Nature of the image depends on where the object is placed. | Image formed is always virtual in nature. |
Page No 238:
Question 4:
Define the following terms for a lens:
(a) optical centre
(b) principal axis
(c) principal focus
(d) focal length
Answer:
(a) The optical centre of a lens is the point on its principal axis through which a ray of light can pass without deviation.
(b) The line joining the centres of the two spheres C1-C2 of which the lenses form a part is called the principal axis of the lens.
(c) The point where a beam of light, parallel to the principal axis, converges after passing through a converging lens or appears to converge in case of a diverging lens is called the principal focus.
(d) The distance between the optical centre and the focus of a lens is called its focal length.
Page No 238:
Question 5:
Why does a concave lens always form a virtual image of an object? Draw a diagram to illustrate this.
Answer:
The rays falling on a concave lens after refraction always diverge. Therefore, no matter where the object is kept, a concave lens always forms a virtual image, which is erect and smaller in size than the object.
Page No 238:
Question 6:
What do you mean by the accommodation of the eye? What gives the eye the power of accommodation?
Answer:
Accommodation of an eye is its ability to alter the focal length of its lens for clearly seeing all the objects within a certain range. This is achieved by changing the thickness of the lens. Hence, its focal length is changed by the action of ciliary muscles.
Page No 238:
Question 7:
How are letters written in the Braille system? Give two examples.
Answer:
Every character in the Braille system is an arrangement of one to six raised dots. Each dot has a numbered position in the Braille cell. These characters make up letters of the alphabet, punctuation marks, numbers and also special characters.
For example:
A is written as
and Y is written as
Page No 238:
Question 8:
What is meant by 'persistence of vision'? How is this property of the eye used in movies?
Answer:
The image of an object is retained on the retina for some time after its formation. This is called 'persistence of vision'. In humans, the image stays for 1/16th of a second. In movies, successive still images are flashed before our eyes at a rate of more than 16 images per second. This allows us to watch the images as a movie.
Page No 239:
Question 1:
A coin kept at the bottom of a glass bowl full of water appears to be higher than it actually is. With the help of a labelled diagram, explain why this happens.
Answer:
Let there be a coin at the bottom of a beaker filled with water. Now, rays of light from point A on the coin will bend away from the normal as they emerge from a denser medium to a lighter medium i.e. from water to air. These rays will then appear to have came from the point A' that is exactly above A. Thus, A' is the virtual image of A. In the same way, each point on the dipped coin will have a corresponding virtual image above it. Due to this phenomenon, the coin appears at a point higher than where it actually is placed.
Page No 239:
Question 2:
'As an object is slowly brought from infinity to the focus of a convex lens, the image travels from the focus to infinity.' Show this with the help of diagrams with five different positions of the object.
Answer:
(i) Object at infinity:
(ii) Object beyond 2F1
(iii) Object at 2F1
(iv) Object between F1 and 2F1
(v) Object at F1
Page No 239:
Question 3:
Draw a labelled diagram of the eye and answer the following questions:
(a) From where does light enter the eye?
(b) Which muscles control the focal length of the eye lens?
(c) Where is the image formed?
(d) Which nerve sends the image signals to the brain?
Answer:
(a) Light enters the eye through the cornea.
(b) Ciliary muscles control the focal length of the eye lens.
(c) The image is formed on the retina of the eye.
(d) Optic nerves send the image signals to the brain.
Page No 239:
Question 4:
Distinguish between myopia and hypermetropia. Draw diagrams to show how these defects can be corrected by using suitable lenses.
Answer:
| Myopia | Hypermetropia |
| A person suffering from this defect can only see the nearby objects clearly. The distant objects will appear blurred to him. | A person suffering from this defect can only see the distant objects clearly, while the nearby objects will appear blurred to him. |
| The image of the distant object is formed before the retina. | The image of the near object is formed behind the retina. |
| The far point is at finite distance. | The far point is at infinity. |
| The near point is at 25 cm. | The near point is beyond 25 cm. |
| It can be corrected using a concave lens. | It can be corrected using a convex lens. |
Page No 239:
Question 1:
Can a glass slab bend different coloured lights by different amounts?
Answer:
No, a glass slab cannot bend different coloured lights by different angles. This is because the two opposite sides of the glass slab are parallel. As a result, the incident ray and the emergent ray are parallel to each other and laterally dispersed. Thus, no dispersion occurs.
Page No 239:
Question 2:
You have seen that a prism splits up light into different colours as it bends different colour lights by different amounts. Why does a glass slab not cause dispersion? (HINT: Observe the path of light in a glass slab−especially what happens to it when it emerges from the slab.)
Answer:
Both faces of a glass slab are parallel. So, when a ray of light falls on one end of the slab, it is laterally displaced; however, all the deviated incident rays meet together to form the white light again while emerging out of the glass slab.
Page No 239:
Question 3:
Suppose a swimming pool is filled with an imaginary liquid which is optically rarer than air. For a person looking at the pool from outside, will the apparent depth of the pool be less than or greater than the real depth? Give reasons.
Answer:
The apparent depth of the pool will be greater than its real depth. This is because the light rays from the bottom of the pool will bend away from the normal, making the apparent point seem deeper than its real depth.
Page No 239:
Question 4:
What type of image is formed when rays of light coming from a point on the object diverge after reflection or refraction−real or virtual?
Answer:
A virtual image is formed when rays of light coming from a point on an object diverge after reflection or refraction. This is because the refracted or reflected rays will never meet each other.
Page No 239:
Question 5:
Will a spectrum be formed if blue light falls on a prism? Give reasons.
Answer:
No, a spectrum will not be formed if blue light falls on a prism. This is because unlike white light, blue light is made up of a single colour; therefore, it will not further break into more colours after bending.
Page No 239:
Question 1:
The diagram shows three different light rays falling on prisms. Complete each diagram to show the refracted and emergent rays. What difference will there be between the three?
Answer:
A prism deviates different colors of light at different angles. The red color deviates least while the violet color deviates the most. White light is made up of its seven constituents colors. When white light passes through a prism, each color deviates at a different angle and a band of seven colors is formed.
Page No 239:
Question 1:
While walking to school, you see a visually-challenged person wanting to cross a busy road. But the traffic is very heavy and nobody is willing to stop for him. What will you do?
Answer:
I will help that person in crossing the road safely. I will wait for the traffic light to become red and then I will take him to the other side of the road with me.
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