NCERT Exemplar Class 10 Science Solutions Chapter 11 The Human Eye and the Colourful World

Question:2

A student sitting on the last bench can read the letters written on the blackboard but is not able to read the letters written in his textbook. Which of the following statements is correct?
(a) The near point of his eyes has receded away
(b) The near point of his eyes has come closer to him
(c) The far point of his eyes has come closer to him
(d) The far point of his eyes has receded away
Ans. (a)

The student sitting on the last bench can read the letters written on the blackboard, but is not able to read the letters written in his textbook.
He is suffering from hypermetropia or farsightedness. He can see distant objects clearly but cannot see nearby objects distinctly.
Therefore, option (a) is correct

Question:3

A prism ABC (with BC as base) is placed in different orientations. A narrow beam of white light is incident on the prism as shown in Figure 11.1. In which of the following cases, after dispersion, does the third colour from the top correspond to the colour of the sky?

(a) (i)
(b) (ii)
(c) (iii)
(d) (iv)
Ans.
Given that the third colour from the top corresponds to the colour of the sky, i.e., blue.
In normal orientation, the third colour from the top is yellow (as shown in the figure).
The third colour from the bottom is blue.

So we have to rotate the prism upside down so that blue comes at the third position from the top. We can do this as follows:

Therefore, option (ii) is correct

Question:4

At noon the sun appears white as
(a) light is least scattered
(b) all the colours of the white light are scattered away
(c) blue colour is scattered the most
(d) red colour is scattered the most
Ans. (a)
At noon, the sun appears white because the light from the sun is directly overhead us. The light travels a relatively shorter distance during the noon.
The scattering of light is reduced as the distance is reduced. So, the sun appears white as only a little of the blue and violet colours are scattered.
Therefore, option (a) is correct

Question:5

Which of the following phenomena of light are involved in the formation of a rainbow?
(a) Reflection, refraction and dispersion
(b) Refraction, dispersion and total internal reflection
(c) Refraction, dispersion and internal reflection
(d) Dispersion, scattering and total internal reflection
Ans. (c)
A rainbow is caused by dispersion, refraction and internal reflection of sunlight by tiny water droplets.
The tiny droplets of water present in the atmosphere act as a prism after rainfall for the rays coming from the Sun. Subsequently, the sunlight after striking the surface of these droplets gets refracted and dispersed into its seven colours. After this, the light rays are subjected to total internal reflection (complete reflection of a ray of light within a medium from the surrounding surfaces back into the medium).
At that point, the rays are again refracted when they come out of the water droplet.
Hence, a rainbow formation is the combined effect of the refraction, dispersion, and total internal reflection of light.

Therefore, option (c) is correct

Question:6

Twinkling of stars is due to atmospheric
(a) dispersion of light by water droplets
(b) refraction of light by different layers of varying refractive indices
(c) scattering of light by dust particles
(d) internal reflection of light by clouds
Ans. (b)
The light of stars undergoes refraction continuously on entering the Earth's atmosphere before it reaches the Earth.
Now we know that the apparent position of the star changes or fluctuates, so the path of rays of light coming from the star goes on changing slightly. That is why the amount of starlight entering the eye flickers. The star sometimes appears brighter, and at other times, it appears fainter. This makes the star twinkle for us.
Hence, the twinkling of a star is due to atmospheric refraction of light of stars.
Therefore, option (b) is correct

Question:7

The clear sky appears blue because
(a) blue light gets absorbed in the atmosphere
(b) ultraviolet radiations are absorbed in the atmosphere
(c) violet and blue lights get scattered more than lights of all other colours by the atmosphere
(d) light of all other colours is scattered more than the violet and blue colour lights by the atmosphere
Ans. (c)
The amount of scattering is inversely proportional to the fourth power of wavelength.
The wavelengths of colours are given below:

Based on the above data, blue and violet colours scatter the most.
Hence, the clear sky appears blue due to the scattering of sunlight.
Therefore, option (c) is correct.

Question:8

Which of the following statements is correct regarding the propagation of light of different colours of white light in air?
(a) Red light moves fastest
(b) Blue light moves faster than green light
(c) All the colours of the white light move with the same speed
(d) Yellow light moves with the mean speed as that of the red and the violet light
Ans. (c)
The propagation of light of different colours of white light in air or vacuum moves with the same speed but different wavelengths and frequencies.
The velocity of light is dependent on the medium, but it does not depend on the frequency and wavelength of light.
Therefore, option (c) is correct

Question:9

The danger signals installed at the top of tall buildings are red in colour. These can be easily seen from a distance because among all other colours, the red light
(a) is scattered the most by smoke or fog
(b) is scattered the least by smoke or fog
(c) is absorbed the most by smoke or fog
(d) moves fastest in air
Ans. (b)
The amount of scattering is inversely proportional to the fourth power of wavelength.
The wavelength of red colour is the largest, so red colour is scattered the least by smoke or fog.
Hence, the danger signals installed at the top of tall buildings are red in colour.
Thus, it can be easily seen from a distance. This is the reason red is associated with danger, and red is used as a stop signal as well.
Therefore, option (b) is correct

Question:10

Which of the following phenomena contributes significantly to the reddish appearance of the sun at sunrise or sunset?
(a) Dispersion of light
(b) Scattering of light
(c) Total internal reflection of light
(d) Reflection of light from the Earth
Ans. (b)
The amount of scattering is inversely proportional to the fourth power of wavelength.
The wavelength of red colour is the largest, so red colour is scattered the least by smoke or fog.
The reddish appearance of the sun at sunrise or sunset is due to the least scattering of red light.
Therefore, option (b) is correct

Question:11

The bluish colour of water in deep sea is due to
(a) the presence of algae and other plants found in water
(b) reflection of sky in water
(c) scattering of light
(d) absorption of light by the sea
Ans. (c)
The amount of scattering is inversely proportional to the fourth power of wavelength.
The wavelength of blue colour is the lowest, so blue colour is scattered the most.
Hence, the bluish colour of water in the deep sea is due to the scattering of light. The very fine particles in water scatter mainly blue light.
The red, green, orange and yellow, having higher wavelengths, get absorbed more easily by water than blue because of their shorter wavelength.
Therefore, option (c) is correct

Question:12

When light rays enter the eye, most of the refraction occurs at the
(a) crystalline lens
(b) outer surface of the cornea
(c) iris
(d) pupil
Ans. (b)
At the point when the light rays enter the eye through a thin membrane, it forms a transparent bulge on the front surface of the eyeball, called the cornea.
Most refraction in the eye happens when light rays travel through the cornea.
Therefore, option (b) is correct

Question:13

The focal length of the eye lens increases when eye muscles
(a) are relaxed and lens becomes thinner
(b) contract and lens becomes thicker
(c) are relaxed and lens becomes thicker
(d) contract and lens becomes thinner
Ans. (a)
When eye muscles are relaxed and become thinner, the focal length of the eye lens increases.
Similarly, when eye muscles are contracted, the focal length decreases.
The sharp image of the distant object is formed at the retina by adjusting the focal length of the eyelens. This enables us to focus accurately on distant objects.

Therefore, option (a) is correct

Question:14

Which of the following statement is correct?
(a) A person with myopia can see distant objects clearly
(b) A person with hypermetropia can see nearby objects clearly
(c) A person with myopia can see nearby objects clearly
(d) A person with hypermetropia cannot see distant objects clearly
Ans. (c)
Hypermetropia - farsightedness.
Myopia - short-sightedness.
A person with myopia can see nearby objects clearly.
A person with hypermetropia can see distant objects clearly.
Therefore, option (c) is correct

Question:16

A student sitting at the back of the classroom cannot read the letters written on the blackboard clearly. What advice will a doctor give to her? Draw ray diagram for the correction of this defect.
Ans.
Myopia - short-sightedness.
In myopia, a person can see nearby objects clearly but cannot see distant objects clearly.
The doctor advises a concave lens of suitable power to bring the image back onto the retina. This is shown as follows:

Question:17

How are we able to see nearby and also the distant objects clearly?
Ans.
The human eye has an eye lens to adjust its focal length. This is known as the power of accommodation.
We are able to see nearby and also distant objects clearly due to the power of accommodation of the eye.
When the muscles are relaxed, the lens becomes thin and its focal length increases.
When the muscles are contracted, the lens becomes thick, and its focal length decreases.
This enables us to see the distant as well as the nearby objects clearly.
This is what makes the eye see different objects at different distances clearly.

Question:18

A person needs a lens of power –4.5 D for correction of her vision.
(a) What kind of defect in vision is she suffering from?
(b) What is the focal length of the corrective lens?
(c) What is the nature of the corrective lens?
Ans.
(a) The power of the lens is given as negative, so she is suffering from myopia.
(b) Power, P = -4.5 D, focal length, f =?
P = 1/f
f = 1/P
f = 1/-4.5 = -0.222m = -22.2cm
(c) If the focal length is positive, then the lens to be used is a convex lens.
If the focal length is negative, then the lens to be used is a concave lens.
The nature of the corrective lens required here is concave.

Question:19

How will you use two identical prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as white light? Draw the diagram.

Ans.
In the normal orientation of a prism, we have:

If we place another prism in front of this prism, we get:

The angles of deflection of both the prisms are equal and opposite. The first prism splits the white light into seven constituent colours, and the second prism combines these colours back to a single ray to give white light as the emergent light.

Question:20

Draw a ray diagram showing the dispersion through a prism when a narrow beam of white light is incident on one of its refracting surfaces. Also indicate the order of the colours of the spectrum obtained.
Ans.
The ray diagram is as follows:

Order of colours from bottom to top: remembering the keyword VIBGYOR
V - Violet, I - Indigo, B - Blue, G - Green, Y - Yellow, O - Orange and R - Red.

Question:21

Is the position of a star as seen by us its true position? Justify your answer.
Ans.
The light of stars undergoes refraction continuously on entering the Earth's atmosphere, before it reaches the Earth. The atmospheric refraction occurs in a medium of gradually changing refractive index.
The atmosphere bends starlight towards the normal, so the apparent position of the star is slightly different from its actual position. So the apparent position of the star changes or fluctuates.
So, the position of a star as seen by us is not its true position.
Thus, the star appears slightly higher than its actual position

Question:22

Why do we see a rainbow in the sky only after rainfall?
Ans.
A rainbow is caused by dispersion, refraction and internal reflection of sunlight by tiny water droplets.
We see a rainbow in the sky only after rainfall due to the dispersion of sunlight by tiny water droplets present in the atmosphere due to rainfall. These droplets act as a prism after rainfall for the rays coming from the Sun. Subsequently, the sunlight after striking the surface of these droplets gets refracted and then gets dispersed into its seven constituent colours. After this, the light rays are subjected to total internal reflection (complete reflection of a ray of light within a medium from the surrounding surfaces back into the medium).
At that point, the rays are again refracted when they come out of the water droplet.
Hence, the rainbow is formed.

Question:23

Why is the colour of the clear sky blue?
Ans.
This is due to the Rayleigh scattering of sunlight.
The amount of scattering is inversely proportional to the fourth power of wavelength.
The wavelength of blue colour is the lowest, so blue colour is scattered the most.
The molecules of air and other fine particles in the atmosphere have a smaller size than the wavelength of visible light. These particles are more effective in scattering light of shorter wavelengths (blue) than light of longer wavelengths (red).

Question:24

What is the difference in colours of the Sun observed during sunrise/sunset and noon? Give explanation for each.
Ans.
At noon, the sun appears white because the light from the sun is directly overhead us. The light travels a relatively shorter distance during the noon. The scattering of light is reduced as the distance is reduced.
Amount of scattering is inversely proportional to the fourth power of wavelength.
So, the sun appears white as only a little of the blue and violet colours are scattered.
The sun looks reddish during sunrise/sunset, because at this stage, rays from the sun have to travel a much larger part of the atmosphere. As the distance increases in this case, the scattering of light increases. So, the red colour, having the largest wavelength, is scattered the least.

Question:26

When do we consider a person to be myopic or hypermetropic? Explain using diagrams how the defects associated with myopic and hypermetropic eyes can be corrected?
Ans.
Myopia - short-sightedness.
A person with myopia can see nearby objects clearly.
Light from a distant object forms an image before it reaches the retina
This is because the eye is too long or the cornea or crystalline lens is too short.

To correct these defects, a concave lens is placed in front of a myopic eye, moving the image back to the retina and clarifying the image as shown below:

Hypermetropia - farsightedness.
A person with hypermetropia can see distant objects clearly.
An image of a nearby object is formed behind the retina.
This is because the eye is too short or the cornea or crystalline lens does not refract the light enough.

To correct this defect, a convex lens is placed in front of a hypermetropic eye.
The image is moved forward and focuses correctly on the retina.

Question:27

Explain the refraction of light through a triangular glass prism using a labelled ray diagram. Hence define the angle of deviation.
Ans.
The refraction of light through a triangular glass prism is shown below

A – Angle of prism
D – Angle of deviation
i – incident angle
r – angle of refraction
e – emergent angle
A ray of light PE enters at the first surface AB (from air to glass).
As light travels from a rarer to a denser medium, it bends towards the normal.
Hence, EF bends towards the normal.
At the second surface AC, the light enters from a denser to rarer medium, so it bends away from the normal.
Hence, FS bends away from the normal.
The angle made by extending the incident ray with the emergent ray is called the angle of deviation

Question:28

How can we explain the reddish appearance of sun at sunrise or sunset? Why does it not appear red at noon?
Ans.
Amount of scattering is inversely proportional to the fourth power of wavelength.
The wavelength of red colour is the largest, so red colour is scattered the least by smoke or fog.
The reddish appearance of the sun at sunrise or sunset is because the wavelength of visible light from the sun near the horizon is greater than the scattering of light by the molecules of air and other fine particles in the atmosphere.
It passes through a larger distance in the Earth's atmosphere and thicker layers of air before reaching our eyes. Most of the blue light (shorter wavelengths) is scattered away by the particles.
So, only red light, being of the highest wavelength, reaches us, which gives a reddish appearance to the sun at sunrise or sunset.
At noon, the sun appears white because the light from the sun is directly overhead us. The light travels relatively shorter distance during the noon.
The scattering of light is reduced as the distance is reduced. So, the sun appears white as only a little of the blue and violet colours are scattered.

Question:29

Explain the phenomenon of dispersion of white light through a glass prism, using suitable ray diagram.
Ans.
When the white light is passed through a pin, it gets reduced to a narrow beam.
When this narrow beam is made to fall on a triangular glass prism and a white screen is held on the other side of the prism, we observe a band of seven colours on the screen. This phenomenon is called dispersion of white light.
Order of colours from bottom to top: remembering the keyword VIBGYOR
V - Violet, I - Indigo, B - Blue, G - Green, Y - Yellow, O - Orange and R - Red.

Amount of scattering is inversely proportional to the fourth power of wavelength.
The red colour (with maximum wavelength) bends the least on passing through the prism, and the violet colour (with minimum wavelength) bends through the maximum angle on passing through the prism. This is why red is at the top and violet is at the bottom.
Thus, the phenomenon of splitting of white light into its constituent seven colours on passing through a glass prism is known as dispersion of light.

Question:30

How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?
Ans.
The refraction takes place in the atmosphere because of the gradually changing refractive index of the different layers of the atmosphere. The layers at the top are optically rare, while the layers at the bottom are optically denser.
When light travels through different layers of the atmosphere, refraction takes place. Since light passes from rarer to denser mediums, it tends to bend towards the normal.
So, the light of stars undergoes refraction continuously on entering the Earth's atmosphere, before it reaches the Earth. The atmosphere bends starlight towards the normal, so the apparent position of the star is slightly different from its actual position. So the apparent position of the star changes or fluctuates.
So, the position of a star as seen by us is not its true position.
Thus, the star appears slightly higher than its actual position because of atmospheric refraction.

As discussed above, the light of stars undergoes refraction continuously on entering the Earth's atmosphere, before it reaches the Earth.
Now we know that the apparent position of the star changes or fluctuates, so the path of rays of light coming from the star goes on changing slightly. That is why the amount of starlight entering the eye flickers. The star sometimes appears brighter, and at other times, it appears fainter. This makes the star twinkle for us.
Hence, the twinkling of a star is due to atmospheric refraction of the light of stars.