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 14 of 15

1. Q261 CBSE 2017
   Using the phenomenon ​of scattering ​of light, explain ​why ​there is ​a difference in ​the colour of the sun as it appears during sunrise and ​at noon.
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   At sunrise the longer path scatters away most blue, so the Sun looks reddish; at noon the shorter path leaves it white.

   The molecules of air and other fine particles in the atmosphere scatter light of shorter wavelengths (blue, violet) much more than light of longer wavelengths (red). At sunrise the sun is near the horizon, so its light passes through a much thicker layer of air — a much longer distance in the atmosphere — before reaching our eyes. Along this path most of the blue light and other shorter wavelengths are scattered away, and the light that reaches us is mainly of longer wavelengths, so the sun appears reddish. At noon the sun is overhead and its light travels a much shorter distance through the atmosphere. Only a little of the blue and violet is scattered away, so the sun appears white.

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2. Q262 CBSE 2017
   State the cause of dispersion of white light by a ​glass prism. How did Newton, using ​two identical ​glass prisms, show that white light is made ​of seven colours ? Draw a ray diagram to show the path of a narrow beam of ​white ​light, through a combination of two identical prisms ​arranged together in inverted position with respect to each other, when it is allowed to fall obliquely on one of the faces of the first ​prism of the combination.
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   The refractive index of glass differs for each colour, so colours bend unequally; Newton's inverted second prism recombined the spectrum into white light.

   Cause of dispersion: the different colours of white light bend through different angles in the prism, because the refractive index of glass (the speed of light in it) is different for different colours; red bends the least and violet the most, so the colours separate into a spectrum. Newton's experiment: he obtained the spectrum of sunlight with one glass prism and placed a second, identical prism in the inverted position in the path of the spectrum. All the colours recombined and white light emerged from the second prism, which showed that white light is made of the seven colours (VIBGYOR). Ray diagram: the first prism disperses the white beam into a fan of colours; the inverted prism bends each colour back through the same angle, so the colours emerge together, parallel to the incident beam, as white light (see the figure).

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3. Q263 CBSE 2017
   What is ‘dispersion of white light’ ​? State its ​cause. Draw ​a ray diagram to show the dispersion ​of white light by a glass ​prism.
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   Dispersion is the splitting of white light into seven colours; it occurs because glass has a different refractive index for each colour.

   Dispersion of white light is the splitting of white light into its seven component colours — violet, indigo, blue, green, yellow, orange and red — when it passes through a transparent medium such as a glass prism. The band of colours obtained is called the spectrum. Cause: different colours of light bend through different angles with respect to the incident ray as they pass through the prism, because the refractive index of glass is different for different colours (greatest for violet, least for red). Red light bends the least and violet the most, so the colours emerge along different paths and separate. Ray diagram: a narrow beam of white light falls obliquely on one face of the prism; it bends towards the base at both faces and emerges as a spread of colours, with red deviated the least (at the top of the spectrum) and violet deviated the most (at the bottom), as in the figure.

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4. Q264 CBSE 2017
   What is ​“dispersion of white light” ​? Draw a labelled diagram ​to illustrate the recombination ​of the ​spectrum of white light. Why is it essential that the two prisms used for the ​purpose should be identical and placed in an inverted position with respect to each other ​?
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   Dispersion splits white light into seven colours; a second identical, inverted prism bends each colour back equally, so white light emerges.

   Dispersion of white light: the splitting of white light into its seven component colours (violet, indigo, blue, green, yellow, orange, red) when it passes through a prism. It happens because different colours bend through different angles: red the least, violet the most. Recombination: a second prism, identical to the first, is placed in an inverted position next to it. The first prism disperses the white light into a spectrum; the colours pass through the second prism, and a beam of white light emerges from it (labelled diagram). Why identical and inverted: the second prism must bend each colour through exactly the same angle as the first, but in the opposite direction, so that the dispersion is undone and all the colours come out together, parallel to the original beam, as white light. An identical prism gives the same deviation for every colour, and inverting it reverses that deviation; together the two prisms act like a parallel-sided glass slab.

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5. Q265 CBSE 2017
   A student needs a lens of power $-0.5$ D. ​for ​correcting his vision.

   1. (i)Name the defect of vision he ​is suffering ​from. Write two ​causes of this defect.
   2. (ii)What ​is the nature and focal ​length of the correcting lens ?
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   Myopia, caused by excessive curvature of the eye lens or elongation of the eyeball; the lens is concave, $f=-2$ m.

   (i) The student suffers from myopia (near-sightedness). Two causes: excessive curvature of the eye lens, and elongation of the eyeball. (ii) The correcting lens is a concave (diverging) lens, since its power is negative. Its focal length is $f=\frac{1}{P}=\frac{1}{-0.5\text{D}}=-2$ m, that is, $-200$ cm.

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6. Q266 CBSE 2017
   How does refraction of light take ​place in ​the atmosphere ​? ​Explain the reason why stars appear ​to twinkle and ​the planets do ​not twinkle.
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   Air of varying density refracts light continuously, so a star's light fluctuates and it twinkles; a planet, an extended source, shines steadily.

   Refraction in the atmosphere: The atmosphere is not uniform. Its density, and so its refractive index, is greatest near the ground and decreases with height, and it also changes with temperature (hot air has a lower refractive index than cooler air). Light passing through it therefore travels through a medium of gradually changing refractive index and is refracted continuously; starlight coming down into denser air bends towards the normal and follows a curved path. Because the air keeps moving, the amount of bending keeps changing. Stars twinkle: Because of this refraction, a star is seen slightly away from its actual position. As the atmosphere keeps changing, the path of the starlight varies slightly, so the apparent position of the star fluctuates and the amount of light entering the eye flickers: the star appears brighter at one moment and fainter at the next. Stars are so distant that they act as point sources, so this flicker is seen as twinkling. Planets do not twinkle: Planets are much closer to the Earth and are seen as extended sources, each a collection of a large number of point sources. The variations in the light from the individual points average out, so the total light entering the eye stays nearly steady and there is no twinkling.

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7. Q267 CBSE 2017
   What is ​spectrum of white light ​? Why ​do different coloured rays ​deviate ​at different angles ​on passing through a glass prism ? Explain the method of recombining the different components of ​white ​light.
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   White light splits into VIBGYOR in a prism because each colour has a different refractive index; an inverted identical prism recombines them.

   Spectrum: when a beam of white light passes through a glass prism it splits into its component colours; this is dispersion. The band of coloured components so obtained is called the spectrum of white light. Its colours, in order, are violet, indigo, blue, green, yellow, orange and red (VIBGYOR). Different deviations: light of different colours travels with different speeds in glass, so the refractive index of glass is slightly different for each colour, least for red and greatest for violet. Each colour is therefore bent through a different angle at the faces of the prism: red is deviated the least and violet the most, so the colours emerge along different directions and spread out. Recombination: a second prism, identical to the first, is placed in an inverted position (upside down) close to it, so that all the colours coming out of the first prism pass through it. The second prism deviates each colour by the same amount in the opposite direction, so all the colours emerge together, parallel to the original beam, and recombine to give a beam of white light. This is Newton's experiment.

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8. Q268 CBSE 2017
   A person is unable to see ​distinctly ​the words printed on a newspaper. ​Name ​the defect ​of vision he is suffering ​from. Draw ray ​diagram to illustrate ​this defect. List its two possible causes. Draw a ray diagram to show how this defect may be corrected using a lens of appropriate focal length.
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   The defect is hypermetropia, caused by a long eye-lens focal length or a small eyeball, and corrected by a convex lens.

   The defect is hypermetropia (far-sightedness, or long-sightedness): nearby objects cannot be seen distinctly while distant objects can, because the near point of the eye has moved farther than 25 cm. Ray diagram of the defect: rays from an object at the normal near point $N$ (25 cm) are not converged enough by the eye lens and would meet at a point behind the retina; the nearest object this eye can focus on the retina is at its own near point $N^{\prime }$, farther away. Two possible causes: (i) the focal length of the eye lens is too long (its converging power is too small); (ii) the eyeball has become too small. Correction: a convex (converging) lens of suitable focal length is used in front of the eye. It forms a virtual image of an object at 25 cm at the near point $N^{\prime }$ of the person, so the rays seem to come from $N^{\prime }$ and the eye focuses them on the retina.

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9. Q269 CBSE 2017
   Draw a ​ray diagram to explain ​the ​dispersion of white light. With the help of ​a labelled ray diagram, show how ​a spectrum of ​white light may be recombined.
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   A prism disperses white light into VIBGYOR because each colour bends by a different angle; a second, inverted identical prism recombines it.

   Dispersion: when a narrow beam of white light passes through a glass prism, it splits into its seven component colours, violet, indigo, blue, green, yellow, orange and red (VIBGYOR). The glass has a slightly different refractive index for each colour, so the colours are bent through different angles with respect to the incident ray: red is deviated the least and violet the most. They therefore separate and form a band of colours, the spectrum, with violet nearest the base of the prism (see the first ray diagram). Recombination: a second glass prism, identical to the first, is placed in an inverted position beside it so that the spectrum falls on it. It deviates each colour back by the same amount in the opposite direction, so all the colours recombine and white light emerges from the second prism, parallel to the original beam (see the labelled diagram). This showed Newton that white light is made up of seven colours.

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10. Q270 CBSE 2017
    1. (a)Draw a ray diagram to explain the ​term ​angle of deviation.
    2. (b)​Why do the component ​colours of incident ​white light split into a spectrum while passing through ​a ​glass prism, ​explain.
    3. (c)Draw a labelled ray diagram to show the formation of a rainbow.
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    $\angle D$ is the angle between incident and emergent rays; glass bends each colour differently; sunlight in raindrops forms the rainbow.

    (a) A ray $PE$ falls on face $AB$ of a glass prism and bends towards the normal; it travels through the prism along $EF$ and, on leaving face $AC$ at $F$, bends away from the normal and emerges along $FS$. Because the two faces are inclined, the emergent ray is turned towards the base. The angle between the incident ray produced forward and the emergent ray (produced backward) is called the angle of deviation, $\angle D$ (see the ray diagram, which also marks $\angle i$, $\angle r$, $\angle e$ and the angle of the prism $\angle A$). (b) White light is a mixture of seven colours. The refractive index of glass is different for different colours (light of each colour travels at a slightly different speed in glass), so each colour bends through a different angle with respect to the incident ray as it passes through the prism: red bends the least and violet the most. The colours therefore emerge along different paths and become distinct, forming the spectrum VIBGYOR. This splitting is called dispersion. (c) A rainbow is formed by tiny water droplets in the air after a shower, each acting like a small prism. Sunlight entering a droplet is refracted and dispersed, is reflected internally at the back of the droplet, and is refracted again as it comes out. The colours emerge at slightly different angles, so an observer with the Sun behind sees red on the outer edge and violet on the inner edge of a bow in the sky opposite the Sun (see the labelled ray diagram).

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11. Q271 CBSE 2017
    (a) ​Write the functions of each of the following parts ​of the human ​eye :

    1. (i)Cornea
    2. (ii)​Iris
    3. (iii)Crystalline (Eye) lens
    4. (iv)Ciliary muscles
    5. (v)Retina (b) A person is unable to see distinctly the objects closer ​than 1 m. Name the defect of vision he ​is suffering from. Draw ray diagrams to illustrate the cause of the defect and ​its correction ​by suitable lens.
    Show answer

    Cornea, iris, lens, ciliary muscles and retina each have a set function; the defect is hypermetropia, corrected by a convex lens.

    (a)(i) Cornea: the thin transparent membrane at the front of the eye through which light enters; most of the refraction of the light entering the eye occurs at its outer surface. (a)(ii) Iris: a dark muscular diaphragm that controls the size of the pupil, and so regulates the amount of light entering the eye. (a)(iii) Crystalline (eye) lens: a transparent, flexible convex lens that provides the finer adjustment of focal length needed to focus objects at different distances on the retina, where a real, inverted image is formed. (a)(iv) Ciliary muscles: they hold the eye lens and change its curvature, and hence its focal length (relaxed: lens thinner, for distant objects; contracted: lens thicker, for near objects). This is accommodation. (a)(v) Retina: the light-sensitive screen on which the image is formed; its light-sensitive cells produce electrical signals, which the optic nerve carries to the brain. (b) The person suffers from hypermetropia (far-sightedness, or long-sightedness): the near point has moved from about 25 cm out to 1 m. Cause: the focal length of the eye lens is too long, or the eyeball has become too small, so light from a nearby object (at 25 cm) is brought to a focus behind the retina (first ray diagram). Correction: a convex (converging) lens of suitable power, which makes light from an object at 25 cm appear to come from the near point at 1 m, so that the eye can focus it on the retina (second ray diagram). For a lens close to the eye this needs $f\approx +33$ cm, a power of about $+3$ D.

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12. Q272 CBSE 2017
    (a) ​A student suffering from myopia is not able to see distinctly the objects placed beyond 5 m. List two possible reasons due to which this defect of vision may have ​arisen. With the help of ray diagrams, explain

    1. (i)why the student is unable to see distinctly the objects placed beyond 5 ​m from his ​eyes.
    2. (ii)the type of the ​corrective lens used to restore proper vision ​and how this defect is corrected by the use of this lens. (b) If, in ​this case, the numerical value of the focal length of the corrective lens is ​5 m, find the power of the lens as per the new Cartesian sign convention.
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    A concave lens with $f=-5$ m corrects the myopia (far point 5 m); its power is $-0.2$ D.

    (a) Possible reasons: (1) excessive curvature of the eye lens (too short a focal length); (2) elongation of the eyeball. (i) The far point of the student's eye is $5$ m. Rays from an object at $5$ m are focused on the retina, but rays from an object beyond $5$ m are brought to a focus in front of the retina and reach it as a blurred patch, so such objects are not seen distinctly. (ii) A concave (diverging) lens of suitable power is used. It spreads out the rays from a distant object so that they appear to come from the far point, $5$ m away; the eye can focus such rays on the retina, so the image is formed on the retina and proper vision is restored. (b) For the concave lens $f=-5$ m, so $P=\frac{1}{f}=\frac{1}{-5}=-0.2$ D.

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13. Q273 CBSE 2017
    1. (a)Define angle of deviation. ​Why ​do different components of white light split up ​into ​spectrum when it passes through a glass prism ? How can this spectrum be recombine ?
    2. (b)What is ​a rainbow ? ​State two essential conditions for the observation of ​rainbow in the ​sky.
    Show answer

    Glass bends each colour differently, giving dispersion; an inverted identical prism recombines it; a rainbow needs the Sun behind the observer.

    (a) Angle of deviation: the angle between the direction of the incident ray and the direction of the emergent ray when light passes through a prism (the angle between the incident ray produced forward and the emergent ray produced backward). White light is a mixture of colours, and light of each colour travels at a different speed in glass, so the refractive index of glass is different for each colour. Each colour is therefore bent through a different angle by the prism, red the least and violet the most, and the colours emerge along different paths and spread into a spectrum (dispersion). The spectrum can be recombined by placing a second, identical prism in an inverted position (upside down) close to the first; it bends each colour back, and white light emerges from it (Newton's experiment). (b) A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by the dispersion of sunlight by tiny water droplets in the atmosphere: each drop, acting like a small prism, refracts and disperses the sunlight, reflects it internally and refracts it again as it comes out. Two essential conditions: 1. The Sun must be behind the observer (the rainbow forms in the direction opposite to the Sun). 2. There must be water droplets in the air in front of the observer, lit by the sunlight, as just after a rain shower.

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14. Q274 CBSE 2017
    Select ​the ​diagram in which the path of ​a ray passing ​through a glass ​prism is correctly drawn

    1. (a)A
    2. (b)B
    3. (c)C
    4. (d)D
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    (a) A

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15. Q275 CBSE 2017
    Study ​the ​following diagram and select the option ​having ​correctly ​marked angles

    1. (a)$\angle i$, $\angle A$, $\angle D$
    2. (b)$\angle i$, $\angle r$, $\angle D$
    3. (c)$\angle i$, $\angle e$, $\angle A$
    4. (d)$\angle r$, $\angle e$, $\angle D$
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    (a) $\angle i$, $\angle A$, $\angle D$

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16. Q276 CBSE 2017
    A student traces the path ​of a ray of light through a glass prism for different angles of ​incidence. He analysis each diagram and draws the following conclusion : I. On entering ​prism, the light ray bends towards its base. II. Light ray suffers ​refraction at the point of incidence and point of emergence while passing through the prism. III. Emergent ​ray bends at certain angle to the direction of the incident ray. IV. While ​emerging ​from the prism, ​the light ray bends towards the vertex of the prism. Out of the above inferences, the correct ones are :

    1. (a)I, II and III
    2. (b)I, III and IV
    3. (c)II, III and IV
    4. (d)I and IV
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    (a) I, II and III

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17. Q277 CBSE 2017
    In ​the following diagram ​the ​correctly marked ​angles ​are ​:

    1. (a)$\angle \mathrm{A}$ and $\angle e$
    2. (b)$\angle i$, $\angle \mathrm{A}$ and $\angle \mathrm{D}$
    3. (c)$\angle \mathrm{A}$, $\angle r$ and $\angle e$
    4. (d)$\angle \mathrm{A}$, $\angle r$ and $\angle \mathrm{D}$
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    (a) $\angle \mathrm{A}$ and $\angle e$

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18. Q278 CBSE 2017
    In ​the ​following diagram, ​the ​correctly marked ​angles ​are

    1. (a)All
    2. (b)Only $\angle i$ and $\angle A$
    3. (c)$\angle i$, $\angle r$ and $\angle A$
    4. (d)$\angle i$, $\angle A$ and $\angle D$
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    (c) $\angle i$, $\angle r$ and $\angle A$

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19. Q279 CBSE 2017
    In which of ​the following diagrams is the path of ​a ​ray ​of light ​passing ​through a ​glass prism ​shown correctly ?

    1. (a)I
    2. (b)II
    3. (c)III
    4. (d)IV
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    (b) II

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20. Q280 CBSE 2017
    Study the ​following ray ​diagram : In this diagram, ​the angle of incidence, the angle of ​emergence and ​the angle of deviation ​respectively have been ​represented by

    1. (a)y, p, z
    2. (b)x, q, z
    3. (c)p, y, z
    4. (d)p, z, y
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    (c) p, y, z

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