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.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 11 of 15

1. Q201 CBSE 2020
   The near point ​of the eye of a person ​is 50 cm. Find the nature and power of ​the corrective lens ​required by the person to enable him ​to see clearly ​the objects placed at 25 cm. from ​the eye.
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   The person has hypermetropia and needs a convex lens of focal length $+50$ cm and power $+2$ D.

   The person has hypermetropia, so the corrective lens is a convex (converging) lens. With $u=-25$ cm and $v=-50$ cm, $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=\frac{1}{50}$, so $f=+50$ cm $=+0.5$ m and the power is $P=\frac{1}{f}=+2$ D.

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2. Q202 CBSE 2020
   A person may suffer from ​both myopia and hypermetropia defects.

   1. (a)​What is this condition ​called ?
   2. (b)When does it happen ​?
   3. (c)Name the type of lens often required by the persons suffering from this defect. ​Draw labelled ​diagram of such lenses.
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   The condition is presbyopia, which occurs with ageing; bifocal lenses (concave above, convex below) are often required.

   (a) The condition is called presbyopia. (b) It happens with ageing, when the power of accommodation of the eye decreases due to the gradual weakening of the ciliary muscles and the diminishing flexibility of the eye lens. (c) Such persons often require bifocal lenses: the upper portion is a concave lens (for distant vision) and the lower portion is a convex lens (for near vision), as shown in the labelled diagram.

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3. Q203 CBSE 2020
   Which defect ​of the ​eye is ​known as far-sightedness ​? When does this defect arise ? State two reasons. How ​is this defect corrected ​?
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   Far-sightedness is hypermetropia, caused by a long eye-lens focal length or a small eyeball; a convex lens corrects it.

   Far-sightedness is hypermetropia. The defect arises when a person can see distant objects clearly but cannot see nearby objects distinctly: the near point of the eye moves farther away than the normal 25 cm, because light from a nearby object is focused behind the retina. Two reasons: (i) the focal length of the eye lens is too long (its converging power has decreased); (ii) the eyeball has become too small, so the retina is too close to the eye lens. It is corrected by using spectacles with a convex (converging) lens of appropriate power, which supplies the extra focusing power needed to form the image on the retina.

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4. Q204 CBSE 2020
   List three ​common refractive ​defects ​of vision. Suggest the ​ways of correcting ​these ​defects.
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   Myopia is corrected by a concave lens, hypermetropia by a convex lens and presbyopia by bi-focal lenses.

   The three common refractive defects of vision and their correction: (i) Myopia (near-sightedness): distant objects are not seen clearly because their image forms in front of the retina. It is corrected by a concave (diverging) lens of suitable power. (ii) Hypermetropia (far-sightedness): nearby objects are not seen clearly because light from them would focus behind the retina. It is corrected by a convex (converging) lens of suitable power. (iii) Presbyopia: with age the power of accommodation decreases and the near point recedes. It is corrected by bi-focal lenses, with a concave upper part for distant vision and a convex lower part for near vision (a convex lens alone where only near vision is affected). These defects can also be corrected with contact lenses or by surgery.

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5. Q205 CBSE 2020
   1. (a)How is a rainbow ​formed ​?
   2. (b)Why ​do stars twinkle ​?
   3. (c)Why do the sky ​appear ​dark instead of blue to an astronaut ?
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   Raindrops refract, disperse and internally reflect sunlight; atmospheric refraction makes stars twinkle; with no air above the atmosphere nothing scatters light.

   (a) A rainbow is formed by tiny water droplets suspended in the air after a rain shower, which act like small prisms. Sunlight entering a droplet is refracted and dispersed into its colours, is reflected internally at the back of the droplet, and is refracted again as it comes out. Because of the dispersion and the internal reflection, different colours reach the observer’s eye, and the rainbow is seen in the direction opposite the sun. (b) Stars twinkle because of atmospheric refraction. Starlight is refracted continuously as it passes through layers of air whose refractive index keeps changing with the changing physical conditions of the atmosphere. A star is so far away that it acts as a point source, so its apparent position keeps shifting slightly and the amount of its light entering the eye flickers, and the star appears to twinkle. (c) The sky looks blue from the earth because the particles of the atmosphere scatter sunlight, blue light most of all. An astronaut is above the atmosphere, where there is no air (no medium) to scatter sunlight, so no scattered light reaches the eyes from the sky and it appears dark.

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6. Q206 CBSE 2020
   How will you use ​two identical glass prisms so that a ​narrow beam of ​white ​light incident on one prism emerges out ​of the second ​prism as white light ? Draw and label the ray ​diagram.
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   Place a second identical prism in an inverted position next to the first; it bends each colour back, so white light emerges.

   Place the second prism, identical to the first, in an inverted position (upside down) with respect to the first prism and close to it, so that the band of colours from the first prism falls on it. The first prism disperses the narrow beam of white light into its colours (violet bent the most, red the least); the inverted prism bends each colour back through the same angle in the opposite direction, so all the colours emerge together, parallel to the incident beam, and recombine into white light on the screen. The labelled ray diagram shows white light, ${\text{P}}_1$, the inverted ${\text{P}}_2$, the rays R and V, the emergent white light and the screen.

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7. Q207 CBSE 2020
   1. (a)​List two causes ​of hypermetropia.
   2. (b)​Draw ray diagrams ​showing ​(i) ​a hypermetropic ​eye and ​(ii) its correction using suitable optical device.
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   Hypermetropia is caused by a too-long focal length of the eye lens or too small an eyeball; a convex lens corrects it.

   (a) Causes of hypermetropia: (i) the focal length of the eye lens is too long, that is, the power of the eye lens has decreased; (ii) the eyeball has become too small, so the retina is too close to the eye lens. (b) (i) In a hypermetropic eye, rays from an object at N, the near point of a normal eye (25 cm), are converged by the eye lens to a point behind the retina, so a nearby object looks blurred; the near point N′ of this eye is farther than 25 cm (first ray diagram). (ii) It is corrected with a convex (converging) lens of suitable power placed in front of the eye. The lens forms a virtual image of the object at N at the point N′, so the rays entering the eye appear to come from N′ and are focused on the retina (second ray diagram).

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8. Q208 CBSE 2020
   1. (a)With ​the ​help of labelled ray ​diagram ​show the path followed by a narrow beam of monochromatic light when it passes through a glass prism.
   2. (b)What would happen ​if this beam ​is replaced by a narrow ​beam of white light ​?
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   A monochromatic ray is deviated towards the base without splitting; white light splits into seven colours, VIBGYOR (dispersion).

   (a) The incident ray PE bends towards the normal as it enters the prism at E, travels through the glass along EF, and bends away from the normal as it leaves at F, emerging along FS. The emergent ray is bent towards the base of the prism and is deviated from the direction of the incident ray through the angle of deviation $D$. Labels: incident ray, refracted ray, emergent ray, angle of prism $\angle A$, angle of incidence $\angle i$, angle of refraction $\angle r$, angle of emergence $\angle e$, angle of deviation $\angle D$. A monochromatic beam is deviated but does not split. (b) The beam of white light splits into its seven component colours, violet, indigo, blue, green, yellow, orange and red (VIBGYOR), forming a spectrum. This is dispersion: violet is deviated the most and red the least.

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9. Q209 CBSE 2020
   A student ​uses spectacles ​of focal length $-2.5$ m.

   1. (a)Name the defect of vision ​he is suffering from.
   2. (b)Which lens ​is used for the correction of this defect ​?
   3. (c)​List two main causes of developing this defect.
   4. (d)Compute ​the power of this lens.
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   Myopia, corrected by a concave lens; causes are excessive curvature of the eye lens and elongation of the eyeball; $P=-0.4$ D.

   (a) Myopia (near-sightedness): the lens has a negative focal length, so it is a concave lens, the lens used to correct myopia. (b) A concave (diverging) lens of suitable power. (c) (i) Excessive curvature of the eye lens; (ii) elongation of the eyeball. (d) $P=\frac{1}{f}=\frac{1}{-2.5\text{m}}=-0.4$ D.

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10. Q210 CBSE 2020
    1. (a)State the ​relation between colour of scattered light and size of the ​scattering particle.
    2. (b)The apparent position of an object, ​when seen through the ​hot air, fluctuates or wavers. ​State ​the basic cause of this ​observation.
    3. (c)Complete the path of white light when it passes through ​two identical prisms placed as shown :

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    Finer particles scatter shorter wavelengths; hot air's changing refractive index makes objects waver; the inverted second prism recombines the colours into white light.

    (a) The colour of the scattered light depends on the size of the scattering particles: very fine particles, such as air molecules, scatter mainly light of shorter wavelengths (blue, violet), while larger particles scatter light of longer wavelengths (towards red); if the particles are large enough, all colours are scattered about equally and the scattered light looks white. (b) Atmospheric refraction by air whose physical conditions keep changing: the hot air is optically rarer than the cooler air above it, and its temperature, and so its refractive index, keeps fluctuating. Light from the object is refracted by varying amounts, so its apparent position fluctuates. (c) Prism 1 disperses (splits) the white light into a spectrum of seven colours, VIBGYOR, bending violet the most and red the least. The identical, inverted prism 2 bends each colour back by the same amount, so the colours recombine and white light emerges from prism 2, parallel to the incident beam, and falls on the screen as white light (see the diagram).

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11. Q211 CBSE 2020
    (a) A person is suffering from both myopia and ​hypermetropia.

    1. (i)​What kind of lenses can correct this defect ?
    2. (ii)How are ​these lenses prepared ? (b) A person needs a lens of ​power $+3$ D for correcting his near vision and $-3$ ​D for correcting his distant vision. Calculate the focal lengths of the ​lenses required ​to correct these ​defects.
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    Bifocal lenses correct both defects; the focal lengths are $+33.3$ cm for $+3$ D and $-33.3$ cm for $-3$ D.

    (a) (i) Bifocal lenses. (ii) A bifocal lens consists of both a concave and a convex lens: its upper portion is a concave lens, which corrects distant vision, and its lower portion is a convex lens, which corrects near vision. (b) Using $f=\frac{1}{P}$: for $P=+3$ D, $f=\frac{1}{3}$ m $=+33.3$ cm (a convex lens, for near vision); for $P=-3$ D, $f=-\frac{1}{3}$ m $=-33.3$ cm (a concave lens, for distant vision).

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12. Q212 CBSE 2020
    Give reasons :

    1. (a)Red ​colour is selected for danger ​signals.
    2. (b)​The ​sky appears dark in space.
    3. (c)The time ​difference ​between actual sunset and apparent sunset is about 2 minutes.
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    Red is scattered least; space has no air to scatter light; atmospheric refraction shows the Sun about 2 minutes after actual sunset.

    (a) Red light is scattered the least by fog or smoke, so a red signal can be seen in the same colour from a long distance. (b) In space there is no atmosphere, so there is no scattering of sunlight; no scattered light reaches the eye from the sky, so the sky appears dark. (c) Because of atmospheric refraction: sunlight bends towards the Earth as it passes through the atmosphere, so the Sun appears above the horizon even after the actual sunset, for about 2 minutes.

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13. Q213 CBSE 2020
    1. (a)Name two defects of vision. Mention two main causes of ​each defect.
    2. (b)A person ​uses ​spectacles ​having lenses of focal ​length $-2.0$ m. Determine the nature ​and ​power ​of the lenses used in his spectacles.
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    Myopia and hypermetropia with two causes each; $P=-0.5$ D, so the spectacle lenses are concave (diverging).

    (a) Myopia (near-sightedness), in which the image of a distant object forms in front of the retina; its causes are excessive curvature of the eye lens (its focal length becomes too short) and elongation of the eyeball. Hypermetropia (far-sightedness), in which the image of a nearby object forms behind the retina; its causes are too long a focal length of the eye lens and an eyeball that has become too small. (Presbyopia, caused by the weakening of the ciliary muscles and the diminishing flexibility of the eye lens, may be named instead of either.) (b) $P=\frac{1}{f}=\frac{1}{-2.0\text{m}}=-0.5$ D. The focal length and the power are negative, so the lenses are concave (diverging) lenses of power $-0.5$ D; they correct myopia.

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14. Q214 CBSE 2020
    1. (a)A student is ​unable ​to see the words written on the blackboard placed at a distance of approximately 5 m from him clearly. Name the ​defect of vision the student ​is suffering from. State the possible causes ​of this defect and explain in brief the method of correcting this defect.
    2. (b)Why ​does the Sun appear reddish during sunrise and sunset ? Will this phenomenon be observed by an astronaut on the moon ? Give reasons to justify your answer.
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    Myopia, corrected by a concave lens; the Sun looks reddish as blue light is scattered away, but not on the Moon (no atmosphere).

    (a) The student suffers from myopia (near-sightedness): distant objects are not seen clearly because their image forms in front of the retina. Causes: (i) excessive curvature of the eye lens, or (ii) elongation of the eyeball. Correction: spectacles with a concave (diverging) lens of suitable power; it makes the rays from a distant object diverge as if they came from the far point of the eye, so that the eye lens brings the image onto the retina. (b) At sunrise and sunset, light from the Sun near the horizon travels through much thicker layers of air, a longer distance in the atmosphere. Most of the blue and other short-wavelength light is scattered away by the fine particles of air, while red light, which is scattered least, reaches our eyes, so the Sun appears reddish. No, an astronaut on the moon would not observe this: the moon has no atmosphere, so no scattering of light takes place, and the Sun appears white against a black sky.

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15. Q215 CBSE 2019
    What happens to the image distance in ​the normal ​human ​eye ​when we ​decrease the distance of an object, ​say 10 m ​to ​1 m ? Justify your answer.
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    It stays the same, because the image always forms on the retina; the lens adjusts its focal length instead.

    The image distance remains the same. In the eye the image is always formed on the retina, so the image distance is the distance between the eye lens and the retina, which is fixed. When the object is brought from 10 m to 1 m, the ciliary muscles contract, the eye lens becomes thicker and its focal length decreases (accommodation), so the image of the nearer object is still formed on the retina.

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16. Q216 CBSE 2019
    What is atmospheric refraction ​? ​List ​two phenomena ​which can ​be explained on ​the basis of atmospheric ​refraction.
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    Bending of light by the atmosphere; it explains the twinkling of stars and advance sunrise and delayed sunset.

    Atmospheric refraction is the refraction (bending) of light by the Earth's atmosphere, whose optical density (refractive index) changes gradually from layer to layer because the density and temperature of the air change. Two phenomena explained by it: (i) The twinkling of stars; (ii) Advance sunrise and delayed sunset: the Sun is seen about $2$ minutes before it actually rises and for about $2$ minutes after it actually sets. (Other correct examples, such as stars appearing higher than they really are, also earn credit.)

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17. Q217 CBSE 2019
    List ​two causes of ​presbyopia. Draw ​labelled diagram ​of a lens ​used ​for the ​correction of this defect of vision.
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    Weak ciliary muscles and a less flexible eye lens cause presbyopia; a bifocal lens (upper concave, lower convex) corrects it.

    Causes of presbyopia: (i) gradual weakening of the ciliary muscles; (ii) diminishing flexibility of the eye lens (both with age). Lens used for correction: a bifocal lens. Its upper portion is a concave lens, for distant vision, and its lower portion is a convex lens, for near vision (reading); the labelled diagram is at step 4. When only near vision is affected, a convex lens of suitable power is used.

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18. Q218 CBSE 2019
    Define the term power of accommodation. Write the ​modification in the curvature ​of the ​eye ​lens ​which enables ​us to see the nearby objects clearly ​?
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    It is the eye lens's ability to adjust its focal length; for near objects its curvature increases and it becomes thicker.

    Power of accommodation is the ability of the eye lens to adjust its focal length. To see nearby objects clearly, the ciliary muscles contract, the curvature of the eye lens increases (the lens becomes thicker), and its focal length decreases, so the image still forms on the retina.

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19. Q219 CBSE 2019
    Write ​the ​structure of eye ​lens ​and state ​the role of ciliary ​muscles in the human eye.
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    A transparent, biconvex, jelly-like lens; the ciliary muscles change its curvature and focal length to focus near and distant objects.

    Structure: the eye lens is a transparent, convex (biconvex) lens made of a fibrous, jelly-like material. It is held in position behind the pupil by suspensory ligaments attached to the ciliary muscles. Role of the ciliary muscles: they change the curvature of the eye lens and hence its focal length (the power of accommodation). When they relax, the lens becomes thin and its focal length increases, so distant objects are seen clearly; when they contract, the lens becomes thicker and its focal length decreases, so nearby objects are seen clearly.

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20. Q220 CBSE 2019
    Why is the colour of ​the ​clear ​sky blue ?
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    Air molecules scatter blue light far more strongly than red, and the scattered blue reaches our eyes from all over the sky.

    The molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light. Such particles scatter light of shorter wavelengths (the blue end of the spectrum) much more strongly than light of longer wavelengths (the red end); red light has a wavelength about $1.8$ times that of blue light. So, as sunlight passes through the atmosphere, its blue part is scattered strongly in all directions. This scattered blue light enters our eyes from every part of the sky, and the clear sky looks blue.

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