CBSE Class 10 Science The Human Eye and the Colourful World Sure Shot Questions Set 02

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CHAPTER - 11
THE HUMAN EYE AND THE COLOURFUL WORLD
ASSIGNMENTS QUESTIONS SET - 2

MULTIPLE CHOICE QUESTIONS

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) - 0.5 D

1. The person cannot see objects beyond 2 m, so the far point of the eye is at 2 m. This is myopia.
2. The corrective lens must form the image of a distant object at 2 m, so it is a concave lens with focal length f = -2 m.
3. Power P = \( \frac{1}{f} \) = \( \frac{1}{-2} \) = -0.5 D.
4. Hence, the correct option is (b).

Teacher's Note:
a) Myopia is corrected by a concave lens, whose power is always negative.
b) Power in dioptre = 1 divided by focal length in metres.

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) The near point of his eyes has receded away

1. He can see distant objects (the blackboard) clearly, so his far point is normal.
2. He cannot see nearby objects (the text book) clearly, which is hypermetropia.
3. In hypermetropia the near point moves farther away from the eye than 25 cm.
4. Hence, the correct option is (a).

Teacher's Note:
a) Hypermetropia means a far-sighted eye: distant objects are clear, near objects are blurred.
b) Do not confuse this with myopia, where the far point comes closer.
c) The defect is corrected by a convex lens.

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 below Figure. In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky?
(a) (i)
(b) (ii)
(c) (iii)
(d) (iv)

Answer: (b) (ii)

1. The colour of the sky is blue.
2. In the spectrum VIBGYOR, blue is the third colour counted from violet (violet, indigo, blue).
3. Light bends towards the base of the prism, and violet bends the most. So the violet end of the spectrum lies on the side of the base.
4. Blue is the third colour from the top only when the base BC is at the top, which is the case in figure (ii).
5. Hence, the correct option is (b).

Teacher's Note:
a) Red deviates the least and violet the most, so violet always lies on the base side of the spectrum.
b) Count from the violet end: violet, indigo, blue.

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) light is least scattered

1. At noon the Sun is overhead, so its light travels the shortest path through the atmosphere.
2. Very little of the light is scattered on this short path, so the light reaches us nearly unchanged and the Sun appears white.
3. Hence, the correct option is (a).

Teacher's Note:
a) A shorter path through the air means less scattering.
b) At sunrise and sunset the path is longest, so the Sun looks red.

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) Refraction, dispersion and internal reflection

1. Sunlight is refracted when it enters a water droplet and is dispersed into its colours.
2. The coloured light is internally reflected at the back of the droplet.
3. It is refracted again when it leaves the droplet, which spreads the colours further.
4. Hence, the correct option is (c).

Teacher's Note:
a) Remember the order: refraction, dispersion, internal reflection, refraction.
b) A rainbow needs the Sun behind the observer and water droplets in front.

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) refraction of light by different layers of varying refractive indices

1. Starlight passes through layers of the atmosphere whose refractive index keeps changing.
2. The light is refracted again and again, so the apparent position and brightness of the star keep changing.
3. This flickering is seen as twinkling.
4. Hence, the correct option is (b).

Teacher's Note:
a) Twinkling is caused by atmospheric refraction, not by scattering.
b) A star is a point source, so a small change in its path changes its brightness noticeably.

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) violet and blue lights get scattered more than lights of all other colours by the atmosphere

1. Air molecules scatter short wavelengths much more strongly than long wavelengths.
2. Violet and blue have the shortest wavelengths, so they are scattered the most.
3. Our eyes are more sensitive to blue than to violet, so the sky looks blue.
4. Hence, the correct option is (c).

Teacher's Note:
a) Scattering is inversely related to the fourth power of wavelength.
b) Blue is not absorbed; it is scattered in all directions.

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) All the colours of the white light move with the same speed

1. In air (nearly vacuum) all colours of light travel at the same speed, about 3 × 10⁸ m/s.
2. The speed differs between colours only inside a material such as glass, which causes dispersion.
3. Hence, the correct option is (c).

Teacher's Note:
a) Colours separate in glass because their speeds differ there, not in air.
b) Do not mix up speed in a medium with speed in vacuum.

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) is scattered the least by smoke or fog

1. Red light has the longest wavelength among visible colours.
2. Scattering is least for long wavelengths, so red light is scattered the least by smoke or fog.
3. Hence, red light travels farthest and is seen from a long distance, and the correct option is (b).

Teacher's Note:
a) Least scattering means the light stays in a straight line and travels farther.
b) This is also why red is used for danger lights and traffic stop signals.

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) Scattering of light

1. At sunrise and sunset sunlight has to travel a long path through the atmosphere.
2. Most of the blue and shorter wavelengths are scattered away on this path.
3. Mainly red light reaches our eyes, so the Sun appears red.
4. Hence, the correct option is (b).

Teacher's Note:
a) The cause is scattering by air molecules, not dispersion.
b) A long atmospheric path removes the blue end of the light.

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) scattering of light

1. Water molecules scatter the shorter wavelengths of sunlight more than the longer ones.
2. Blue light is scattered the most and comes back to our eyes, so deep water looks blue.
3. Hence, the correct option is (c).

Teacher's Note:
a) The reflected sky is only a small part of the effect; scattering is the main cause.
b) Scattering of blue light also explains the colour of the sky.

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) outer surface of the cornea

1. Light enters the eye from air into the cornea, where the change in refractive index is the largest.
2. So most of the bending of light occurs at the outer surface of the cornea.
3. The crystalline lens only makes the fine adjustment of focal length.
4. Hence, the correct option is (b).

Teacher's Note:
a) The cornea does most of the bending; the lens does the focusing adjustment.
b) The iris controls the size of the pupil and does not refract light.

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) are relaxed and lens becomes thinner

1. When the ciliary muscles relax, the eye lens becomes thin.
2. A thin lens has less curvature, so its focal length increases.
3. This lets the eye see distant objects clearly.
4. Hence, the correct option is (a).

Teacher's Note:
a) Relaxed muscles: thin lens, long focal length, distant vision.
b) Contracted muscles: thick lens, short focal length, near vision.

14. Which of the following statement 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) A person with myopia can see nearby objects clearly

1. A myopic person is near-sighted: nearby objects are clear and distant objects are blurred.
2. A hypermetropic person is far-sighted: distant objects are clear and nearby objects are blurred.
3. So only statement (c) is correct.
4. Hence, the correct option is (c).

Teacher's Note:
a) Myopia means near-sighted and hypermetropia means far-sighted.
b) Check each statement against these two meanings before choosing.

SHORT ANSWER QUESTIONS

15. Draw ray diagrams each showing (i) myopic eye and (ii) hypermetropic eye.

Answer:
1. (i) Myopic eye: parallel rays from a distant object are focused in front of the retina, as shown in the first figure.
2. (ii) Hypermetropic eye: parallel rays are focused behind the retina, as shown in the second figure.

Teacher's Note:
a) Label the eye lens, the retina and the position of the focus.
b) Draw the focus in front of the retina for myopia and behind it for hypermetropia.

16. A student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. What advice will a doctor give to her? Draw ray diagram for the correction of this defect.

Answer:
1. The student cannot see distant objects clearly, so she is suffering from myopia (near-sightedness).
2. The doctor will advise her to wear spectacles with a concave lens of suitable power.
3. The concave lens diverges the rays slightly, so the eye lens forms the image exactly on the retina, as shown in the figure.

Teacher's Note:
a) Myopia needs a diverging (concave) lens.
b) The power of the lens depends on the far point of the eye.

17. How are we able to see nearby and also the distant objects clearly?

Answer:
1. The eye lens is flexible and its curvature is changed by the ciliary muscles.
2. For distant objects the ciliary muscles relax, the lens becomes thin and its focal length increases, so the image forms on the retina.
3. For nearby objects the ciliary muscles contract, the lens becomes thick and its focal length decreases, so the image again forms on the retina.
4. This ability of the eye to adjust its focal length is called accommodation.

Teacher's Note:
a) The image distance is fixed, so the focal length must change.
b) The ability to adjust is called accommodation.

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:
1. (a) The power is negative, so the lens is concave. She is suffering from myopia (near-sightedness).
2. (b) Focal length f = \( \frac{1}{P} \) = \( \frac{1}{-4.5} \) m = -0.22 m (about -22 cm).
3. (c) The corrective lens is a concave (diverging) lens.

Teacher's Note:
a) Negative power means a concave lens and positive power means a convex lens.
b) Convert the focal length to metres before using f = 1/P.
c) Give the sign of the focal length in the final answer.

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:
1. Place the second prism in an inverted position next to the first prism, so that their refracting faces are parallel.
2. The first prism splits the white light into its seven colours.
3. The second prism bends the colours in the opposite direction and brings them together again.
4. The colours recombine and the beam emerges from the second prism as white light, as shown in the figure.

Teacher's Note:
a) The second prism must be inverted, not placed in the same position.
b) The two prisms undo each other, which shows that white light is made of seven colours.

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:
1. A narrow beam of white light falls on one refracting face of the prism and splits into seven colours, as shown in the figure.
2. Red light bends the least and violet light bends the most.
3. From the top (the side away from the base) to the bottom, the order of the colours is Red, Orange, Yellow, Green, Blue, Indigo and Violet (R, O, Y, G, B, I, V).

Teacher's Note:
a) Violet bends the most and lies nearest to the base of the prism.
b) The sequence VIBGYOR is read from the base side.

21. Is the position of a star as seen by us its true position? Justify your answer.

Answer:
1. No, the apparent position of a star is slightly different from its true position.
2. Starlight enters the atmosphere and passes through layers of increasing refractive index, so it bends towards the normal again and again.
3. Because of this atmospheric refraction, the star appears slightly higher than its actual position.

Teacher's Note:
a) The cause is atmospheric refraction.
b) The effect is greater near the horizon.

22. Why do we see a rainbow in the sky only after rainfall?

Answer:
1. After rain, tiny water droplets remain suspended in the air.
2. These droplets act like small prisms: sunlight is refracted and dispersed in them, internally reflected, and refracted again as it leaves.
3. The dispersed colours reach the eye of an observer who has the Sun behind, so a rainbow is seen only after rainfall.

Teacher's Note:
a) A rainbow needs water droplets and sunlight together.
b) The Sun must be behind the observer.

23. Why is the colour of the clear sky blue?

Answer:
1. The atmosphere contains air molecules and very fine particles that are smaller than the wavelength of visible light.
2. These scatter the shorter wavelengths (blue and violet) much more strongly than the longer wavelengths (red).
3. The scattered blue light reaches our eyes from all directions, so the clear sky looks blue.

Teacher's Note:
a) Blue light has a shorter wavelength than red light, so it is scattered more.
b) Without an atmosphere the sky would look dark.

24. What is the difference in colours of the Sun observed during sunrise/sunset and noon? Give explanation for each.

Answer:
1. At sunrise and sunset the Sun appears reddish. Its light travels a long path through the atmosphere, so most of the blue and shorter wavelengths are scattered away and mainly red light reaches us.
2. At noon the Sun appears white. Its light travels the shortest path through the atmosphere, so very little of it is scattered and all colours reach us together.

Teacher's Note:
a) The difference is only in the length of the path through the air.
b) Less scattering gives white light and more scattering gives red light.

LONG ANSWER QUESTIONS

25. Explain the structure and functioning of Human eye. How are we able to see nearby as well as distant objects?

Answer:
1. The human eye is a nearly spherical organ. Light enters through the transparent cornea, which does most of the refraction.
2. Behind the cornea is the iris, a dark muscular diaphragm, which controls the size of the pupil and so the amount of light entering the eye.
3. The eye lens, held by the ciliary muscles, focuses the light on the retina.
4. The retina is a light-sensitive screen with rod and cone cells, which change the image into electrical signals.
5. The optic nerve carries these signals to the brain, which interprets them as the upright image of the object.
6. For distant objects the ciliary muscles relax and the lens becomes thin, and for nearby objects they contract and the lens becomes thick. This adjustment is called accommodation.

Teacher's Note:
a) Name the cornea, iris, pupil, lens, ciliary muscles, retina and optic nerve.
b) The image on the retina is real and inverted; the brain makes it appear upright.
c) The least distance of distinct vision is 25 cm and the far point is at infinity.

26. When do we consider a person to be myopic or hypermetropic? Explain using diagrams how the defects associated with myopic and hypermetropic eye can be corrected?

Answer:
1. A person is myopic (near-sighted) when he can see nearby objects clearly but cannot see distant objects clearly. The image is formed in front of the retina, because the eyeball is too long or the lens is too curved.
2. A person is hypermetropic (far-sighted) when he can see distant objects clearly but cannot see nearby objects clearly. The image is formed behind the retina, because the eyeball is too short or the lens is too thin.
3. Myopia is corrected by using a concave lens of suitable power, which diverges the rays so that the image falls on the retina (first figure).
4. Hypermetropia is corrected by using a convex lens of suitable power, which converges the rays so that the image falls on the retina (second figure).

Teacher's Note:
a) Contrast both defects: position of the image, cause and type of lens.
b) Draw the lens in front of the eye in each correction diagram.

27. Explain the refraction of light through a triangular glass prism using a labelled ray diagram. Hence define the angle of deviation.

Answer:
1. In the figure, a ray EF is incident on the face AB of the prism at point E. It enters the glass, which is denser, so it bends towards the normal.
2. Inside the prism the ray travels along EF and meets the second face AC at F. Here it goes from glass to air and bends away from the normal.
3. The emergent ray bends towards the base BC of the prism.
4. The angle between the direction of the incident ray and the direction of the emergent ray is called the angle of deviation (D in the figure).

Teacher's Note:
a) Light bends towards the normal entering glass and away from it when leaving.
b) The deviation is always towards the base of the prism.

28. How can we explain the reddish appearance of sun at sunrise or sunset? Why does it not appear red at noon?

Answer:
1. At sunrise and sunset the Sun is near the horizon, so its light has to travel a very long path through the atmosphere to reach us.
2. On this long path the blue light and other shorter wavelengths are scattered away by air molecules and fine particles.
3. The red light, which has the longest wavelength, is scattered the least, so it reaches our eyes and the Sun appears reddish.
4. At noon the Sun is overhead and its light travels a much shorter path, so only a little blue and violet is scattered.
5. Hence, all colours reach us nearly together and the Sun appears white and not red.

Teacher's Note:
a) Long path means more scattering of short wavelengths.
b) Red travels farthest because it is scattered the least.

29. Explain the phenomenon of dispersion of white light through a glass prism, using suitable ray diagram.

Answer:
1. When a narrow beam of white light falls on a glass prism, it splits into a band of seven colours. This splitting is called dispersion.
2. Different colours of light have different speeds in glass, so the prism bends each colour by a different amount.
3. Red bends the least and violet bends the most, so the colours separate as shown in the figure.
4. The band of colours obtained is called the spectrum, and its order from the top is R, O, Y, G, B, I, V.

Teacher's Note:
a) Dispersion happens because each colour has a different refractive index in glass.
b) The spectrum order is VIBGYOR from the base side.

30. How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?

Answer:
1. The atmosphere has layers of air with different densities, so its refractive index increases from the top towards the ground.
2. Light from a star passes through these layers and bends towards the normal at each layer, so the apparent position of the star changes slightly. This is atmospheric refraction.
3. The atmosphere is not steady, so the amount of bending keeps changing and the star appears to flicker. This is the twinkling of stars.
4. A star is very far, so it acts as a point source and the light reaching us changes noticeably.
5. A planet is nearer and acts as an extended source made of many point sources. The changes in light from these points average out, so the planets do not twinkle.

Teacher's Note:
a) Atmospheric refraction explains twinkling and the apparent position of stars.
b) A point source twinkles, an extended source does not.