NCERT Exemplar Solution for CBSE Class 10 Science Chapter: Human Eye and Colourful World
In this article you will get CBSE Class 10 Science chapter 11, Human Eye and Colourful World: NCERT Exemplar Problems and Solutions
Multiple Choice Questions
Question 1. A person cannot see distinctly objects kept beyond 2 m. This defect can be corrected by using a lens of power
(a) +0.5 D
(b) ─0.5 D
(c) +0.2 D
(d) ─0.2 D
Answer. (b)
Explanation: As the person with myopia uses a concave lens.
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 text book. 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
Answer. (a)
Explanation: The student is suffering from hypermetropia wherein distant images are clear but the nearby images are blurry.
NCERT Solutions for CBSE Class 10 Science
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. In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky?
Answer. (ii)
Explanation: Inverted position of prism (II) gives the same color at third top as that of sky.
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
Answer. (a)
Explanation: Sun is directly over head and sunlight travel relatively shorter distance causing only little of the blue and violet colours to be scattered.
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
Answer. (c)
Explanation: When Sunlight falls on tiny water droplets, rainbow is formed by dispersion, refraction and internal reflection of sunlight.
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
Answer. (b)
Explanation: The twinkling of a star is due to atmospheric refraction of light of stars.
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
Answer. (c)
Explanation: The molecules in the air scatter blue light more than red light.
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
Answer. (c)
Explanation: Speed of light is a constant value regardless of its color.
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
Answer. (b)
Explanation: Red colour is scattered the least by smoke or fog due to its largest wavelength.
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
Answer. (b)
Explanation: Red colour is scattered the least by due to its largest wavelength.
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
Answer. (c)
Explanation: The fine particles in water scatter mainly blue light having shortest wavelength.
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
Answer. (b)
Explanation: The transparent, anterior portion of outer layer of eyeball is termed as cornea which causes most of the bending of incident light rays, i.e. refraction, to make them converge which in turn causes image formation on retina.
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
Answer. (a)
Explanation: The focal length of the eye lens increases when eye muscles relaxed and becomes thinner.
Question 14. Which of the following statements 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
Answer. (c)
Explanation: Nearsightedness, or myopia is inability to see the far located objects clearly.
(a) +0.5 D
(b) ─0.5 D
(c) +0.2 D
(d) ─0.2 D
Answer. (b)
Explanation: As the person with myopia uses a concave lens.
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 text book. 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
Answer. (a)
Explanation: The student is suffering from hypermetropia wherein distant images are clear but the nearby images are blurry.
NCERT Solutions for CBSE Class 10 Science
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. In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky?
Answer. (ii)
Explanation: Inverted position of prism (II) gives the same color at third top as that of sky.
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
Answer. (a)
Explanation: Sun is directly over head and sunlight travel relatively shorter distance causing only little of the blue and violet colours to be scattered.
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
Answer. (c)
Explanation: When Sunlight falls on tiny water droplets, rainbow is formed by dispersion, refraction and internal reflection of sunlight.
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
Answer. (b)
Explanation: The twinkling of a star is due to atmospheric refraction of light of stars.
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
Answer. (c)
Explanation: The molecules in the air scatter blue light more than red light.
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
Answer. (c)
Explanation: Speed of light is a constant value regardless of its color.
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
Answer. (b)
Explanation: Red colour is scattered the least by smoke or fog due to its largest wavelength.
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
Answer. (b)
Explanation: Red colour is scattered the least by due to its largest wavelength.
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
Answer. (c)
Explanation: The fine particles in water scatter mainly blue light having shortest wavelength.
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
Answer. (b)
Explanation: The transparent, anterior portion of outer layer of eyeball is termed as cornea which causes most of the bending of incident light rays, i.e. refraction, to make them converge which in turn causes image formation on retina.
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
Answer. (a)
Explanation: The focal length of the eye lens increases when eye muscles relaxed and becomes thinner.
Question 14. Which of the following statements 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
Answer. (c)
Explanation: Nearsightedness, or myopia is inability to see the far located objects clearly.
Short Answer Type Questions
Question 15. Draw ray diagrams each showing
(i) myopic eye and
(ii) hypermetropic eye.
Answer.
(i) Myopic eye: myopia is caused by elongation of eyeball which results in focusing of image in front of retina. Thus, the distant images are blurry but the nearby images are clear.
(ii) Hypermetropic eye : Loss of flexibility is characterized by focusing of image behind the retina. In this case, distant images are clear but the nearby images are blurry and the condition is termed as hypermetropia.
Question 16. A student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. What advice will a doctor given to her? Draw ray diagram for the correction of this defect.
Answer.
Student ishaving myopia which is caused by elongation of eyeball resulting in focusing of image in front of retina. Thus, the distant images are blurry but the nearby images are clear.
Wearing of concave lens diverge the light rays to help the image to be focused on the retina; myopia is corrected using the concave lens.
Question 17. How are we able to see nearby and also the distant objects clearly?
Answer.
Eyes have ability to adjust its focal length which is known as power of accommodation and we can see nearby and also the distant objects clearly. Relaxation of muscles makes the lens thinner and its focal length increases to make us see the distant objects clearly. Contraction of ciliary muscles increases the curvature of the eye lens and makes the eye lens thicker. Consequently, the focal length of the eye lens decreases. This enables us to see nearby objects 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?
Answer.
(a) The negative power of lens suggests that she must be suffering from myopia.
(b) Power, P = ─4.5 D, focal length, f = ?
Now, P = 1/ f
⟹ f = 1/ P = 1/ −4.5 = −0.222m = − 22.2 cm
(c) Myopia is corrected using concave or divergent lens.
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.
Answer.
When the second identical prism placed in an inverted position with respect to the first prism, the condition is fulfilled.
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.
Answer.
Question 21. Is the position of a star as seen by us its true position? Justify your answer.
Answer.
No, light of stars enters the earth’s atmosphere and undergoes refraction continuously before reaching the earth. The atmospheric refraction takes place in under gradually changing refractive index. Since, star light is bent towards the normal; star appears slightly higher than the actual position.
Question 22. Why do we see a rainbow in the sky only after rainfall?
Answer.
Raindrops present in atmosphere after rainfall serve as prism and refract, disperse and internally reflect the incident sunlight followed by final refraction. The whole sequence of dispersion and internal reflection leads to formation of rainbow after rainfall in sky.
Question 23. Why is the colour of the clear sky blue?
Answer.
It is caused by Rayleigh scattering of sunlight. The molecules in the air scatter blue light more than red light as blue light has shortest wavelength as compared to that of red light which has about 1.8 times greater than blue light.
Question 24 What is the difference in colours of the sun observed during sunrise/sunset and noon? Give explanation for each.
Answer.
During sunrise/sunset, the sun looks reddish because at this stage, Sun rays travel longer distance in atmosphere and the red light having largest wavelength is scattered the least. Sun is directly over head and sunlight travel relatively shorter distance causing only little of the blue and violet colors to be scattered.
Long Answer Type Questions
Question. 25 Explain the structure and functioning of the human eye. How are we able to see nearby as well as distant objects?
Answer.
Structure of human eye: Cornea is the transparent window of eye that serves in refraction of most of the light rays towards the retina.Iris is the coloured part of the eye that controls the amount of light by regulating the pupil size. Pupil is a hole shaped opening in the middle of the eye. Retina is the innermost layer of the eye and contains an outer pigmented layer and an inner nervous layer. It has photo receptors (rods and cones). Optic nerve consists of nerve fibres from innermost layer of retina and serves to transmit impulse to brain. Eye lens is made up of a fibrous, jelly-like material and transparent structure. It serves to form an inverted real image of the object on the retina. Vitreous humour is clear, semi-solid structure that supports the eye ball.
Functioning of human eye: Light enters the eye through pupil and then the eye lens converges these light rays on the retina. An inverted real image of the object is formed on the retina and image-electric signals are to brain via the optic nerves. The brain reconstruct erect image of objects and we can see the objects.
We are able to see nearby as well as distant objects due to the eye’s ability to adjust its focal length which is known as power of accommodation. Relaxation of muscles makes the lens thinner and its focal length increases to make us see the distant objects clearly. Contraction of ciliary muscles increases the curvature of the eye lens and makes the eye lens thicker. Consequently, the focal length of the eye lens decreases. This enables us to see nearby objects clearly.
Question. 26 When do we consider a person to be myopic and hypermetropic? Explain using diagrams how the defects associated with myopic and hypermetropic eye can be corrected?
Answer.
Myopia: This eye defect is caused by elongation of eyeball which results in focusing of the image in front of retina. This causes formation of blurry images of the distant images but the nearby images are clear.
Correction of Myopia: Wearing of concave lens diverge the light rays to help the image to be focused on the retina, hence myopia is corrected using the concave lens.
Hypermetropia: Loss of flexibility or too short eyeball causes focusing of image behind the retina. In both cases, distant images are clear but the nearby images are blurry and the condition is termed as hypermetropia.
Correction of hypermetropia:Wearing a convex lens (plus powered) in front of a hypermetropic eye moves the image forward and focuses it 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.
Answer.
The light ray PE enters from air to glass (rarer to denser medium) at surface AB and therefore, bends towards the normal. The refracted ray EF now exits from glass and enters air (from denser to rarer medium) and therefore, bends away from the normal. Extrapolation of incident and emergent ray gives 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?
Answer.
It is caused by Rayleigh scattering of sunlight. The molecules in the air scatter blue light more than red light as blue light has shortest wavelength as compared to that of red light which has about 1.8 times greater than blue light. Sky appears blue. During sunrise/sunset, the sun looks reddish because at this stage, Sun rays travel longer distance in atmosphere and blue and green light (having shortest wavelength) of visible spectrum is scattered away fully while the red light having largest wavelength is scattered the least.
Sun is directly over head and sunlight travel relatively shorter distance causing only little of the blue and violet colors to be scattered.
Question. 29 Explain the phenomenon of dispersion of white light through a glass prism, using suitable ray diagram.
Answer.
Splitting of light into its constituent colors is referred to as dispersion. When a narrow beam of white light falls on a triangular glass prism, light of different colors have different refractive indices in glass. However, the speed of light is same irrespective of its colors. Different refractive indices of different colors of light lead to their different bending pattern. This causes splitting of white light into light of seven colors called as VIBGYOR. V= violet, I= indigo, B=blue, G=green, Y= yellow, O=orange, R=red.
The light of red color bends the least on passing through the prism and appears at the top while violet colour bends through maximum angle and appears at bottom.
Question. 30 How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?
Answer.
Optical densities of air particles change with changing height. This in turn produce different pattern of refraction of light by the earth’s atmosphere. The continuously changing physical conditions air produce different pattern of refraction of light stars. The greater the refraction is, the brighter the star look. The lesser the refraction is, the dimmer the star look. This different refraction pattern through different air layers makes stars to look twinkling.
The planets are present much closer to the earth as compared to stars which makes planets to look like an extended sources of light, not like a point-sized sources of light as stars appear. This results in average of least and maximum refraction as zero making the planets to look stationary not twinkling as stars.