Class 10 Science · Chapter 10

The Human Eye and the Colourful World

281 Class 10 Science questions on The Human Eye and the Colourful World from the NCERT textbook, NCERT Exemplar and CBSE board papers 2017–2026, each with its answer and a step-by-step solution.

NCERT exercises

Exercises (12) Intext Questions (Page 164) (4)

NCERT Exemplar exercises

Multiple Choice Questions (14) Short Answer Questions (10) Long Answer Questions (6)

CBSE Class 10 Science previous year papers, with answers →

Questions · page 10 of 15

Q181 CBSE 2023

A person is suffering from an eye defect in which the far point of the eye is nearer than infinity. Identify the defect. List two main causes of this defect. Draw a ray diagram to show how this defect is corrected by using a suitable lens.

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Myopia; causes are excessive curvature of the eye lens and elongation of the eyeball; a concave lens corrects it.
The defect is myopia (near-sightedness): the person sees nearby objects clearly but not distant ones, because the image of a distant object is formed in front of the retina. Two causes: (i) excessive curvature of the eye lens, which makes its focal length too short; (ii) elongation of the eyeball. Correction: a concave (diverging) lens of suitable power is placed in front of the eye. It makes the parallel rays from a distant object diverge as if they came from the far point F, and the eye lens then brings them to a focus on the retina (see the ray diagram). For spectacles worn close to the eye, the focal length of the lens equals the distance of the far point, with a negative sign.

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Q182 CBSE 2023

Draw a labelled diagram of a common type of bifocal lens. State the function of each of its parts. Name the defect for the correction of which it is used and state its main cause.

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Bi-focal lens: concave upper part for distant vision, convex lower part for near vision; used for presbyopia, which comes with age.
Diagram: a bi-focal lens is a single spectacle lens in two portions; the upper portion is a concave lens and the lower portion is a convex lens (see the labelled diagram). Functions: the upper, concave (diverging) part facilitates distant vision; the lower, convex (converging) part facilitates near vision, as in reading. Defect: presbyopia. NCERT prescribes bi-focal lenses for a person who suffers from both myopia and hypermetropia, typically an ageing eye that has presbyopia and also cannot see distant objects clearly. Main cause: the gradual weakening of the ciliary muscles and the diminishing flexibility of the eye lens with age, so the power of accommodation decreases and the near point recedes.

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Q183 CBSE 2023
Give reasons for the following :
1. (a)Danger signals installed at airports and at the top of tall buildings are of red colour.
2. (b)The sky appears dark to the passengers flying at very high altitudes.
3. (c)The path of a beam of light passing through a colloidal solution is visible.
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Red is scattered least, so it is seen from far; almost no scattering occurs at high altitude; colloidal particles scatter the beam.
(a) Of all the colours of visible light, red has the longest wavelength, and the scattering of light falls off steeply as the wavelength increases. Red light is therefore scattered the least by the fog, smoke and dust particles in the air, so it travels the farthest and remains clearly visible from a great distance even in hazy weather. That is why danger signals are made red. (b) The blue colour of the sky is sunlight scattered towards us by the molecules and particles of the atmosphere. At very high altitudes the atmosphere is extremely thin and there are almost no molecules or dust particles to scatter the sunlight, so practically no scattered light reaches the passengers and the sky appears dark. (c) The particles of a colloidal solution are large enough to scatter light. As the beam passes through, light is scattered sideways from every point of its path and some of it reaches the eye, so the path of the beam becomes visible. This scattering of light by a colloid is called the Tyndall effect.

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Q184 CBSE 2023
(a) State one important function of the following parts of the human eye :
1. (i)Retina
2. (ii)Pupil (b) State the role of ciliary muscles in focussing objects at varying distances from the eye.
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Retina receives the image, pupil controls the light entering, and ciliary muscles change the lens's focal length to focus at different distances.
(a) (i) Retina: it is the light-sensitive screen at the back of the eyeball on which the eye lens forms a real, inverted image; its cells turn the light into electrical signals that the optic nerve carries to the brain. (ii) Pupil: it regulates and controls the amount of light entering the eye, being made small by the iris in bright light and large in dim light. (b) The ciliary muscles change the curvature and hence the focal length of the eye lens. On contracting, the lens becomes thicker, the focal length decreases and nearby objects are focused on the retina; on relaxing, the lens becomes thinner, the focal length increases and distant objects are focused on the retina. This is called accommodation.

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Q185 CBSE 2023
State reasons for Myopia. With the help of ray diagrams, show the
1. (a)image formation by a myopic eye, and
2. (b)correction of myopia using an appropriate lens.
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Myopia is caused by excessive curvature of the eye lens or an elongated eyeball; a concave lens of suitable power corrects it.
Myopia, or short-sightedness, is the defect in which a person sees nearby objects clearly but cannot see distant objects distinctly. It is caused either by excessive curvature of the eye lens, which makes it converge light too strongly, or by elongation of the eyeball, which places the retina too far behind the lens. (a) Because of either cause, parallel rays from a distant object are brought to a focus **in front** of the retina, so only a blurred patch reaches it (first ray diagram). (b) The defect is corrected by placing a concave, that is diverging, lens of suitable power in front of the eye: it spreads the rays out a little, as though they had come from the far point of the defective eye, and the eye lens then brings them to a sharp focus exactly on the retina (second ray diagram). The focal length of the correcting lens equals the distance of the far point, so its power is negative.

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Q186 CBSE 2023

What is Tyndall effect ? Give two examples where this phenomenon is observed. Give reason to explain this phenomenon.

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Colloidal particles scatter a light beam in all directions, making its path visible: the Tyndall effect.
Tyndall effect is the scattering of a beam of light by the colloidal particles present in a medium, because of which the path of the beam becomes visible. Two examples: (i) Sunlight entering a dense forest through the canopy, where the path of the sunbeams can be seen; (ii) A fine beam of light entering a dark, smoke-filled room through a small hole, where the path of the beam is visible (the headlight beams of a car in fog are an equally good example). Reason: the colloidal particles are large enough, their size being comparable with the wavelength of light, to scatter the light falling on them in all directions. Some of this scattered light reaches our eyes from the side, so we see the path of the beam. The particles of a true solution are far smaller than the wavelength of light and scatter it far too weakly, so no path is seen there.

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Q187 CBSE 2023
A person is unable to see clearly a poster fixed on a distant wall. He however sees it clearly when standing at a distance of about 2 m from the wall.
1. (a)Draw ray diagram to show the formation of image by his eye lens when he is far away from the wall.
2. (b)List two possible reasons of this defect of vision.
3. (c)Draw ray diagram to show the correction of this defect using appropriate lens.
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Myopia, with a far point about 2 m away, is corrected by a concave lens.
He can see near objects clearly but not distant ones, and his far point is about $2$ m instead of infinity: he has myopia (near-sightedness). (a) Rays from the distant poster arrive parallel, and his eye lens brings them to a focus in front of the retina, so the retina receives only a blurred image (first ray diagram). (b) Two possible reasons: excessive curvature of the eye lens, and elongation of the eyeball. (c) The defect is corrected with a concave (diverging) lens of suitable power placed in front of the eye: it spreads the parallel rays so that they appear to come from his far point, about $2$ m away, and the eye lens then focuses them on the retina (second ray diagram). For a far point of about $2$ m this lens has a focal length of about $- 2$ m, a power of about $- 0.5$ D.

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Q188 CBSE 2023
(a) White light is dispersed into seven visible coloured components by a glass prism. Name the colour which bends
1. (i)the most and
2. (ii)the least. (b) How can the coloured components of white light be recombined after a prism has separated them ? Draw a labelled diagram to justify your answer.
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Violet bends the most and red the least; a second identical prism, inverted, recombines the colours into white light.
(a) (i) Violet bends the most. (ii) Red bends the least. (b) Place a second glass prism, identical to the first, in an inverted position (upside down with respect to the first) in the path of the spread-out colours. The second prism bends each colour back through the same angle that the first prism bent it, so all the colours leave it travelling parallel to one another and to the original beam; they overlap, and a beam of white light emerges from the other side of the second prism. This is Newton's arrangement, shown in the labelled diagram.

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Q189 CBSE 2023
A person keeps the reading material much beyond 25 cm from the eye for comfortable reading. Name the defect of vision he is suffering from. How can it be corrected ? Draw ray diagrams for
1. (i)the defective eye and
2. (ii)its correction.
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Hypermetropia, corrected by a convex lens that makes rays from 25 cm appear to come from the near point.
The person is suffering from hypermetropia (long-sightedness or far-sightedness): the near point of the eye lies beyond the normal 25 cm, so nearby objects cannot be seen distinctly. It arises because the focal length of the eye lens is too long or the eyeball has become too small, so light from a nearby object is focused behind the retina. It is corrected by using a convex (converging) lens of appropriate power, which makes rays from an object at 25 cm appear to come from the near point N′ of the defective eye, so that the eye lens focuses them on the retina. (i) In the defective eye, rays from an object at N (25 cm) meet behind the retina. (ii) With the convex lens in front of the eye, the same rays are brought to a focus on the retina (see the two ray diagrams).

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Q190 CBSE 2023

A person cannot see distinctly the object placed beyond 5 m from his eyes. Name the defect of vision the person is suffering from. Draw a ray diagram to illustrate this defect. List its two possible causes. Name the lens used for the correction of this defect.

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Myopia; causes are excessive curvature of the eye lens and elongation of the eyeball; a concave lens of $f = - 5$ m corrects it.
The person suffers from myopia (near-sightedness): the far point has come in from infinity to 5 m. Ray diagram: rays from an object at the far point, 5 m away, are focused on the retina, but parallel rays from a distant object meet in front of the retina, so the object looks blurred (see the figure). Causes: (i) excessive curvature of the eye lens; (ii) elongation of the eyeball. Correction: a concave (diverging) lens of suitable power, here of focal length $- 5$ m, a power of $- 0.2$ D.

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Q191 CBSE 2023
Draw a labelled diagram to show
1. (i)dispersion of a beam of white light, and
2. (ii)formation of a rainbow.
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A prism splits white light into a spectrum; a raindrop refracts, internally reflects and refracts sunlight to form a rainbow.
(i) Dispersion by a glass prism (first figure): a narrow beam of white light falls on one face of a glass prism. The colours are refracted by different amounts on entering and again on leaving, violet the most and red the least, so the beam splits into a band of seven colours, the spectrum, which falls on a screen with red at the top and violet at the bottom (VIBGYOR read from the bottom up). Labels: white light, glass prism, screen, spectrum, red to violet. (ii) Formation of a rainbow (second figure): sunlight entering a raindrop is refracted and dispersed, internally reflected at the back of the drop, and refracted again as it comes out. Red leaves at about $4 2^{\circ}$ and violet at about $4 0^{\circ}$ to the direction opposite the Sun, so an observer with the Sun behind sees a rainbow with red on the outer edge and violet on the inner edge. Labels: sunlight (white light), raindrop, refraction and dispersion, internal reflection, refraction, red, violet.

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Q192 CBSE 2023
Define the term dispersion of white light. State the colour which bends
1. (i)the most,
2. (ii)the least while passing through a glass prism. Draw a diagram to show the dispersion of white light.
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Dispersion is the splitting of white light into its colours by a prism; violet bends the most and red the least.
Dispersion of white light is the splitting of white light into its component colours (violet, indigo, blue, green, yellow, orange and red, VIBGYOR) when it passes through a prism. It happens because the different colours bend through different angles, the refractive index of glass being different for each colour. (i) Violet bends the most. (ii) Red bends the least. Diagram: a ray of white light falls on one face of a glass prism, splits into the seven colours as it is refracted there, bends towards the base again at the second face, and forms a band on a screen with red at the top and violet at the bottom (see the figure).

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Q193 CBSE 2020
The sky appears dark to passengers flying at very high altitudes mainly because :
1. (a)Scattering of light is not enough at such heights.
2. (b)There is no atmosphere at great heights.
3. (c)The size of molecules is smaller than the wavelength of visible light.
4. (d)The light gets scattered towards the earth.
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(a) Scattering of light is not enough at such heights.

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Q194 CBSE 2020
The image distance from the eye lens in the normal eye when we increase the distance of an object from the eye
1. (a)increases.
2. (b)decreases.
3. (c)remains unchanged.
4. (d)depends on the size of the eyeball.
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(c) remains unchanged.

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Q195 CBSE 2020
Consider the following reasons for the reddish appearance of the sun at the sunrise or the sunset : A. Light from the sun near the horizon passes through thinner layers of air. B. Light from the sun covers larger distance of the earth’s atmosphere before reaching our eyes. C. Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles. D. Light from the sun near the horizon passes through thicker layers of air. The correct reasons are
1. (a)A and C only
2. (b)B, C and D
3. (c)A and B only
4. (d)C and D only
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(b) B, C and D

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Q196 CBSE 2020
Person suffering from cataract has
1. (a)elongated eyeball
2. (b)excessive curvature of eye lens
3. (c)weakened ciliary muscles
4. (d)opaque eye lens
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(d) opaque eye lens

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Q197 CBSE 2020
For question numbers 13 and 14, two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below :
Assertion (A): While passing through a glass prism, the red component of white light deviates the most and the violet component the least.
Reason (R): For glass, the refractive index for violet light is highest and for red light it is lowest.
1. (a)Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).
2. (b)Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).
3. (c)(A) is true, but (R) is false.
4. (d)(A) is false, but (R) is true.
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(d) (A) is false, but (R) is true.

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Q198 CBSE 2020

Why is Tyndall effect shown by colloidal particles? State four instances of observing the Tyndall effect.

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Colloidal particles are large enough to scatter light, so a beam's path becomes visible, as with headlights in fog.
Colloidal particles are large enough to scatter light. When a beam of light passes through a colloid, its particles scatter the light in all directions, and the scattered light reaching our eyes makes the path of the beam visible; this scattering of light by colloidal particles is the Tyndall effect. (The particles of a true solution are too small to scatter light, so it does not show the effect.) Four instances: (1) A fine beam of sunlight entering a smoke-filled dark room through a small hole becomes visible. (2) Sunlight passing through the canopy of a dense forest in misty or foggy conditions forms visible beams. (3) The beams of car headlights are clearly seen in fog. (4) The beam from a projector in a dusty cinema hall is visible. The blue colour of the clear sky and the red colour of the Sun at sunrise and sunset, which also come from scattering of light in the atmosphere, are accepted as instances too; any other correct instance earns credit.

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Q199 CBSE 2020
Differentiate between a glass slab and a glass prism. What happens when a narrow beam of
1. (i)a monochromatic light, and
2. (ii)white light passes through (a) glass slab and (b) glass prism?
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A slab has parallel faces and only shifts a ray sideways; a prism has inclined faces, deviates the ray and disperses white light.
A glass slab has two parallel refracting surfaces, whereas a glass prism has two refracting surfaces inclined to each other (at the angle of the prism). (i) Monochromatic light: (a) through a glass slab it emerges parallel to the incident ray but displaced sideways (lateral displacement); (b) through a glass prism it bends towards the base and emerges in a different direction, that is, it is angularly displaced (deviated) through the angle of deviation. (ii) White light: (a) through a glass slab it is only laterally displaced and emerges parallel to its original direction, still white (no dispersion); (b) through a glass prism it is dispersed into a band of seven colours (VIBGYOR), red being deviated the least and violet the most.

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Q200 CBSE 2020
Draw a labelled diagram to show
1. (i)reddish appearance of the sun at the sunrise or the sunset and
2. (ii)white appearance of the sun at noon when it is overhead.
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Long path at sunrise or sunset scatters away blue, so the sun looks reddish; short path at noon scatters little, so it looks white.
(i) At sunrise or sunset the sun is near the horizon, so its light travels a long distance through the atmosphere, almost parallel to the Earth's surface, before it reaches the observer. Along this long path most of the blue light (shorter wavelengths) is scattered away by the very fine particles of air, so the light that reaches the eye is mainly of longer wavelengths, red and orange, and the sun appears reddish. (ii) At noon the sun is overhead and its light comes straight down through the atmosphere, the shortest path through the air. Only a little of the blue light is scattered, so light of all colours reaches the eye in nearly the same proportion and the sun appears white. The labelled diagram shows the Earth, the atmosphere and the observer; the sun near the horizon with the long path of its light through the atmosphere, blue light scattered away and the sun appearing reddish; and the sun overhead with the short path of its light and the sun appearing white.

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