10th Class NCERT Solution The Human Eye and the Colourful World

Answers to Science Questions

1. The human eye can focus on objects at different distances by adjusting the focal length of the eye lens. This is due to

(a) presbyopia. (b) accommodation. (c) near-sightedness. (d) far-sightedness.

Answer: (b) accommodation.

2. The human eye forms the image of an object at its

(a) cornea. (b) iris. (c) pupil. (d) retina.

Answer: (d) retina.

3. The least distance of distinct vision for a young adult with normal vision is about

(a) 25 m. (b) 2.5 cm. (c) 25 cm. (d) 2.5 m.

Answer: (c) 25 cm.

4. The change in focal length of an eye lens is caused by the action of the

(a) pupil. (b) retina. (c) ciliary muscles. (d) iris.

Answer: (c) ciliary muscles.

5. A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?

Answer: (i) For distant vision, focal length = 1 / power = 1 / (-5.5) ≈ -0.1818 m or -18.18 cm. (ii) For near vision, focal length = 1 / power = 1 / (+1.5) ≈ 0.6667 m or 66.67 cm.

6. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

Answer: The lens required is concave (diverging) to move the far point to infinity. Power = 1 / focal length = 1 / (-0.80) = -1.25 dioptres.

7. Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.

Answer: [Imagine a diagram with a convex lens in front of the eye, bending light to focus on the retina.] Power = 1 / (1 / 0.25 - 1 / 1) = 1 / (4 - 1) = 1 / 3 ≈ +3.33 dioptres.

8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

Answer: A normal eye cannot focus on objects closer than 25 cm because the ciliary muscles cannot adjust the lens enough to shorten the focal length beyond this limit.

9. What happens to the image distance in the eye when we increase the distance of an object from the eye?

Answer: The image distance remains approximately the same (fixed at the retina), but the eye adjusts the lens focal length to keep the image focused.

10. Why do stars twinkle?

Answer: Stars twinkle due to the refraction of their light by the Earth’s atmosphere, which causes variations in brightness as the air density changes.

11. Explain why the planets do not twinkle.

Answer: Planets do not twinkle because they are closer and appear as extended objects, so the light from different parts averages out the atmospheric effects.

12. Why does the sky appear dark instead of blue to an astronaut?

Answer: The sky appears dark to an astronaut because there is no atmosphere in space to scatter sunlight, which is why we see blue on Earth.