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 12 of 15

1. Q221 CBSE 2019
   A ​glass prism is able to produce ​a ​spectrum ​when white light ​passes ​through it but a rectangular block of same transparent glass does not produce ​any spectrum. Why ?
   Show answer

   Parallel faces of a block undo the bending, so colours recombine into white; a prism's inclined faces add the bending and spread colours.

   White light is a mixture of colours, and glass bends (refracts) each colour through a slightly different angle: violet the most and red the least. This happens when light enters a prism and also when it enters a rectangular glass block. In a rectangular glass block the two refracting faces are parallel. The extent of bending at the second face is equal and opposite to that at the first face, so every colour emerges parallel to the incident ray (angle of emergence = angle of incidence) and hence parallel to the other colours. The colours, only shifted sideways, overlap and recombine into white light, so no spectrum is produced. In a prism the two refracting faces are inclined to each other, so the bending at the second face adds to the bending at the first. The different colours emerge in different directions, spread out and form a spectrum.

   Step-by-step solution →
2. Q222 CBSE 2019
   What happens to a ​beam of white light when it ​gets refracted through a glass prism ? Which colour ​deviates the most and the least after refraction through a ​prism ​? What is likely ​to happen if a second identical prism is placed in an inverted position with respect to the first prism ? Justify your ​answer.
   Show answer

   White light disperses into seven colours, violet deviating most and red least; an inverted identical prism recombines them into white light.

   White light splits into its seven component colours (violet, indigo, blue, green, yellow, orange, red), forming a spectrum; this is called dispersion, and it happens because glass bends each colour through a different angle. Violet deviates the most and red the least. If a second identical prism is placed in an inverted position, the colours recombine and white light emerges from the second prism. Justification: the inverted prism deviates each colour through the same angle as the first prism but in the opposite direction, so all the colours come together again (Newton's experiment), showing that white light is made up of these seven colours.

   Step-by-step solution →
3. Q223 CBSE 2019
   A student ​needs spectacles of ​power $-0.5$ D for ​the ​correction of ​his vision.

   1. (i)Name ​the defect in vision the ​student is suffering from.
   2. (ii)Find the nature and focal length of the ​corrective lens.
   3. (iii)List two causes of this defect.
   Show answer

   Myopia; a concave lens of focal length $f=-200$ cm; causes are excessive curvature of the eye lens and elongation of the eyeball.

   (i) Myopia (near-sightedness). (ii) The power is negative, so the corrective lens is a concave (diverging) lens. Its focal length is $f=\frac{1}{P}=\frac{1}{-0.5\text{D}}=-2$ m $=-200$ cm, that is, a concave lens of focal length $200$ cm. (iii) (a) Excessive curvature of the eye lens; (b) Elongation of the eyeball.

   Step-by-step solution →
4. Q224 CBSE 2019
   What is the cause ​of dispersion of white light ​through a glass ​prism ? Draw a ray ​diagram to ​show the path of light when two identical glass prisms are arranged together in inverted position with respect ​to each other and ​a narrow beam of white light is allowed to fall obliquely on one of the faces of the prisms.
   Show answer

   Dispersion occurs because glass has a different refractive index for each colour; the second, inverted identical prism recombines the colours into white light.

   Cause: White light is made up of seven colours, which travel at different speeds in glass, so the refractive index of glass is different for each colour. Each colour is therefore bent through a different angle on refraction, violet the most and red the least, and because the two refracting faces of a prism are inclined to each other, the colours spread out into a spectrum. This splitting is dispersion. Ray diagram: The first prism splits the narrow beam of white light into a spectrum (red deviated least, violet most). The second, identical prism, placed inverted, bends each colour back through the same angle in the opposite direction, so all the colours emerge parallel to the incident beam and recombine, and white light emerges from the second prism. Label the incident white light, the two prisms, the spectrum (R to V) between them and the emergent white light, with arrows on the rays.

   Step-by-step solution →
5. Q225 CBSE 2019
   What ​is scattering of light ​? Use this phenomenon to explain why

   1. (i)the ​Sun appears reddish at sun-rise, and ​
   2. (ii)​the clear sky appears ​blue.
   Show answer

   Fine particles scatter blue much more than red, so the Sun looks reddish at sunrise and the clear sky looks blue.

   Scattering of light is the throwing of light in all directions when it strikes very small particles of the medium, such as molecules of air and fine dust. Very fine particles scatter light of shorter wavelength (blue) much more strongly than light of longer wavelength (red). (i) At sunrise the Sun is near the horizon, so its light travels a much longer path through the atmosphere. Most of the blue light and other shorter wavelengths are scattered away by the particles on the way, and the light reaching our eyes is mainly red, which has the longest wavelength and is scattered least. So the Sun appears reddish. (ii) The molecules of air and other fine particles in the atmosphere scatter the blue part of sunlight much more strongly than the red, because blue light has a shorter wavelength. This scattered blue light enters our eyes from all parts of the sky, so the clear sky appears blue.

   Step-by-step solution →
6. Q226 CBSE 2019
   What is a ​rainbow? Draw ​a ​labelled diagram to ​show the ​formation ​of ​a rainbow.
   Show answer

   A rainbow is a natural spectrum of seven colours in the sky after rain, formed by dispersion of sunlight in water droplets.

   A rainbow is a natural spectrum of sunlight that appears in the sky after a rain shower, as an arc of seven colours. It is caused by the dispersion of sunlight by tiny water droplets in the atmosphere, which act like small prisms, and it is always formed in the direction opposite to the Sun. Labelled diagram (see the figure): a ray of sunlight (white light) falls on a raindrop; (1) it is refracted and dispersed as it enters the drop; (2) it is reflected internally at the back of the drop; (3) it is refracted and dispersed again as it comes out, so the colours emerge separated, red and violet at different angles, toward an observer who has the Sun behind.

   Step-by-step solution →
7. Q227 CBSE 2019
   What is atmospheric refraction ? Explain with the ​help ​of a labelled diagram ​that ​the position of a star as seen by us is not ​its ​true ​position.
   Show answer

   Atmospheric refraction is the bending of light by air of varying density; it makes a star appear slightly higher than it is.

   Atmospheric refraction is the refraction (bending) of light by the Earth's atmosphere, caused by the variation in the optical density (refractive index) of air: the air near the Earth is denser and the air higher up is rarer. Starlight entering the atmosphere undergoes continuous refraction before it reaches the Earth: passing into denser and denser air, it keeps bending towards the normal. The eye sees the star along the direction in which the light finally enters it, so the apparent position of the star is slightly different from its actual position - slightly higher above the horizon. Labelled diagram: the Earth and an observer; the atmosphere as layers with refractive index increasing towards the Earth; the curved path of starlight from the true position of the star to the eye; and the dashed line of sight traced back to the apparent position of the star, above the true position.

   Step-by-step solution →
8. Q228 CBSE 2019
   When do ​we consider ​a student ​sitting ​in the class to be myopic ? List two causes of this defect. Explain ​using a ray diagram how this ​defect ​of ​eye can be corrected.
   Show answer

   Myopic: sees near objects but not the blackboard; caused by excessive lens curvature or eyeball elongation; corrected by a concave lens.

   (i) A student is considered myopic (near-sighted) when the student can see nearby objects, such as the notebook, clearly but cannot see distinctly the words written on the blackboard: the far point of the eye has come nearer than infinity, so the image of a distant object forms in front of the retina. (ii) Causes: (1) excessive curvature of the eye lens (or of the cornea at the front of the eyeball); (2) elongation of the eyeball. (iii) The defect is corrected with spectacles having a concave (diverging) lens of suitable power, whose focal length equals the distance of the far point. Ray diagram: parallel rays from a distant object fall on the concave lens and diverge as if they came from the far point F; the eye lens then converges them to form the image on the retina (without the lens they would meet in front of the retina).

   Step-by-step solution →
9. Q229 CBSE 2019
   A person is ​unable to see objects distinctly placed within ​50 cm from his eyes.

   1. (a)​Name the defect of vision the person is suffering from and list its two possible ​causes.
   2. (b)Draw a ray diagram to show the defect in ​the above case.
   3. (c)Mention ​the type of lens used by him for the correction of the defect and calculate its power. Assume that the near point for ​the normal eye is 25 cm.
   4. (d)Draw a labelled diagram for the correction of the defect in the ​above case.
   Show answer

   Hypermetropia, corrected by a convex lens with $f=+50$ cm, so its power is $P=+2$ D.

   (a) The person is suffering from hypermetropia (far-sightedness): the near point has moved away to 50 cm. Two possible causes: the focal length of the eye lens is too long (the eye lens has become less converging), or the eyeball has become too small (too short). (b) In the ray diagram of the defect, rays from an object at $N^{\prime }$, 50 cm away, meet on the retina, but rays from an object at $N$, 25 cm away, would meet behind the retina, so its image is blurred. (c) A convex (converging) lens. With $u=-25$ cm and $v=-50$ cm: $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=-\frac{1}{50}+\frac{1}{25}=\frac{1}{50}$, so $f=+50$ cm $=+0.5$ m and $P=\frac{1}{f}=+2$ D. (d) In the labelled correction diagram, a convex lens of power $+2$ D placed in front of the eye bends the rays from an object at $N$ (25 cm) so that they appear to come from $N^{\prime }$ (50 cm); the eye lens then focuses them on the retina.

   Step-by-step solution →
10. Q230 CBSE 2019
    When do ​we consider a ​person to be myopic or hypermetropic ? List two causes of hypermetropia. ​Explain using ​ray ​diagrams how the defect associated with hypermetropic eye ​can be corrected.
    Show answer

    Myopia blurs distant objects, hypermetropia nearby ones; a convex lens makes rays from 25 cm appear to come from the near point.

    Myopia: a person is considered myopic (near-sighted) when he or she can see nearby objects clearly but cannot see distant (far-away) objects distinctly. Hypermetropia: a person is considered hypermetropic (far-sighted) when he or she can see distant objects clearly but has difficulty in seeing nearby objects distinctly; the near point of the eye is farther away than $25$ cm. Two causes of hypermetropia: (i) the focal length of the eye lens is too long; (ii) the eyeball has become too small. Correction: a hypermetropic eye can focus on the retina only objects at its own near point N′, which is farther than $25$ cm (ray diagram of the near point). Rays from an object at the normal near point N ($25$ cm) would meet behind the retina (ray diagram of the defect). A convex (converging) lens of suitable power placed in front of the eye bends the rays from N so that they appear to come from N′ (ray diagram of the correction); the eye lens then forms the image on the retina. The convex lens thus supplies the additional focusing power needed.

    Step-by-step solution →
11. Q231 CBSE 2019
    1. (a)What is scattering of light ? Explain how the colour ​of ​the scattered light depends on the size ​of the scattering particles.
    2. (b)Explain ​the reddish ​appearance of the Sun ​at sunrise or sunset. Why does ​it ​not appear red at noon ?
    Show answer

    Particle size decides the colour scattered; the Sun looks reddish at sunrise and sunset as its light crosses more air, white at noon.

    (a) Scattering of light is the throwing of light in different directions when it strikes very small particles in its path, such as molecules of air, fine dust, smoke or water droplets. The colour of the scattered light depends on the size of the particles: very fine particles (like air molecules) scatter mainly blue light, of shorter wavelength; larger particles scatter light of longer wavelengths (towards red); and if the particles are large enough, all colours are scattered about equally, so the scattered light looks white. (b) At sunrise and sunset the Sun is near the horizon, so its light has to travel a much longer distance through the atmosphere to reach us. Most of the blue light and other shorter wavelengths are scattered away on the way, and the light that reaches our eyes is mainly of longer wavelength (red), so the Sun looks reddish. At noon the Sun is overhead and its light travels a much shorter distance through the air; only a little of the blue is scattered, so the Sun appears white.

    Step-by-step solution →
12. Q232 CBSE 2019
    (a) ​State the role of ciliary muscles present in our eye. (b) Identify the defect of ​vision in each ​of the following cases ​and suggest its corrective measure :

    1. (i)The eye lens ​has become milky ​and ​cloudy.
    2. (ii)The eye lens has excessive curvature.
    3. (iii)The eye lens has ​large focal length (longer than normal).
    4. (iv)Ciliary muscles have weakened.
    Show answer

    Ciliary muscles adjust the lens’s focal length; the defects are cataract (surgery), myopia (concave lens), hypermetropia (convex lens) and presbyopia (convex or bifocal lens).

    (a) The ciliary muscles hold the eye lens and change its curvature, and hence its focal length. 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. This ability of the eye lens to adjust its focal length is called accommodation. (b) (i) Cataract: corrected by cataract surgery, in which the cloudy lens is removed and replaced by an artificial lens. (ii) Myopia (near-sightedness): corrected by a concave (diverging) lens of suitable power. (iii) Hypermetropia (far-sightedness): corrected by a convex (converging) lens of suitable power. (iv) Presbyopia: corrected by a convex lens for near vision, or by bifocal lenses (upper part concave, lower part convex) when distant vision is also affected.

    Step-by-step solution →
13. Q233 CBSE 2019
    1. (a)With the help of diagram explain Isaac Newton’s experiment that led to the idea ​that the sunlight ​is made up of ​seven colours.
    2. (b)​What ​is ​atmospheric refraction ? List two natural phenomena based ​on ​atmospheric refraction.
    Show answer

    Newton's inverted second prism recombined the spectrum into white light; atmospheric refraction causes twinkling of stars and advance sunrise and delayed sunset.

    (a) Newton passed a narrow beam of sunlight through a glass prism and obtained a band of seven colours (VIBGYOR) on a screen: the prism disperses white light. When he passed one colour of this spectrum through a second similar prism, no new colours appeared. He then placed a second prism, identical to the first, in an inverted position next to it (see the diagram). All the colours passed through the second prism, which bent them back, and a beam of white light emerged from its other side. So the seven colours together make white light: sunlight is made up of seven colours. (b) Atmospheric refraction is the refraction of light by the Earth's atmosphere, whose layers of air have gradually changing optical density (refractive index). Two natural phenomena based on it: (i) twinkling of stars; (ii) advance sunrise and delayed sunset - the Sun is seen about $2$ minutes before it actually rises and about $2$ minutes after it actually sets. (Stars appearing higher than they are, and the flattened look of the Sun at sunrise and sunset, are also accepted.)

    Step-by-step solution →
14. Q234 CBSE 2019
    A student has to trace ​the path of ​a ​ray of light ​through a glass prism. List four precautions ​he ​should ​observe for ​better results.
    Show answer

    Keep the prism fixed, fix the pins vertically, keep the angle of incidence between $30^{\circ }$ and $60^{\circ }$ and the pins 5 cm apart.

    Any four of the following: (i) Keep the prism exactly within its traced boundary throughout the experiment; do not move it. (ii) Fix the pins vertically, and look at the feet of the pins (not their heads) while aligning them. (iii) Keep the angle of incidence between $30^{\circ }$ and $60^{\circ }$. (iv) Keep the two pins on each side at least 5 cm apart. (v) Use the protractor correctly, measuring the angles from the normal, and draw thin lines with a sharp pencil. (Any other correct precaution also earns credit.)

    Step-by-step solution →
15. Q235 CBSE 2019
    A student traces the ​path of a ray of light through a glass prism as shown ​in the diagram, but leaves it incomplete ​and ​unlabelled. Redraw ​and complete the diagram. ​Also label ​on it $\angle i$, $\angle e$, $\angle r$ and $\angle D$.

    Show answer

    Join E to F for the refracted ray, draw the normals, and mark $\angle i$, $\angle r$, $\angle e$ and $\angle D$ as defined.

    Completed diagram (see the figure): join the point E, where the incident ray meets the first face, to the point F, where the emergent ray leaves the second face; EF is the refracted ray inside the prism. Draw the normals to the faces at E and F. $\angle i$ (angle of incidence): between the incident ray and the normal at E. $\angle r$ (angle of refraction): between the refracted ray EF and the normal at E, inside the prism. $\angle e$ (angle of emergence): between the emergent ray and the normal at F. $\angle D$ (angle of deviation): between the incident ray produced forward and the emergent ray produced backward, where the two meet.

    Step-by-step solution →
16. Q236 CBSE 2018
    Name ​the ​phenomenon responsible ​for ​the twinkling ​of ​stars in the sky.
    Show answer

    Atmospheric refraction of starlight causes the twinkling of stars.

    Atmospheric refraction, that is, the refraction of starlight by layers of air of changing density in the Earth's atmosphere.

    Step-by-step solution →
17. Q237 CBSE 2018
    Which type of lens is ​used for ​the ​correction ​of myopia ​?
    Show answer

    A concave (diverging) lens of suitable power is used to correct myopia.

    A concave (diverging) lens of suitable power (a lens of negative power) is used to correct myopia.

    Step-by-step solution →
18. Q238 CBSE 2018
    Why does ​the sky ​appear dark ​instead ​of blue to ​an astronaut ?
    Show answer

    There is no atmosphere in space to scatter sunlight, so no scattered light reaches the astronaut and the sky looks dark.

    There is no atmosphere in space, so there are no air molecules or fine particles to scatter sunlight. No scattered light reaches the astronaut's eye from the sky, so the sky appears dark instead of blue.

    Step-by-step solution →
19. Q239 CBSE 2018
    What ​is the ​meaning ​of power of ​accommodation ​of ​human ​eye ?
    Show answer

    It is the ability of the eye lens to adjust its focal length so that near and distant objects are seen clearly.

    The power of accommodation is the ability of the eye lens to adjust (change) its focal length, with the help of the ciliary muscles, so that both nearby and distant objects are seen clearly, their images being formed on the retina.

    Step-by-step solution →
20. Q240 CBSE 2018
    A person needs a lenses of power $+2.0$ D ​for the correction of his vision.

    1. (a)Name ​the defect ​of vision the person ​is ​suffering ​from. Write two causes ​of this defect.
    2. (b)What is the nature and focal length of the corrective lenses ?
    Show answer

    Hypermetropia, caused by a long eye-lens focal length or a small eyeball; the lens is convex, $f=+50$ cm.

    (a) The power is positive, so the corrective lens is converging: the person suffers from hypermetropia (far-sightedness), that is, can see distant objects clearly but not nearby objects. Two causes: (i) the focal length of the eye lens is too long; (ii) the eyeball has become too small. (b) The corrective lens is convex (converging), with focal length $f=\frac{1}{P}=\frac{1}{+2.0\text{D}}=+0.5\text{m}=+50\text{cm}$.

    Step-by-step solution →