NCERT Class 10 Science Chapter 10 Human Eye and Colourful World Exercise Solutions

Subject: Science
Class: 10
Board: NCERT / CBSE

Q1.

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
Explanation: The eye lens changes its focal length to focus clearly on objects at various distances — this ability is called accommodation.

Q2.

The human eye forms the image of an object at its —
(a) cornea (b) iris (c) pupil (d) retina

✅ Answer: (d) Retina
Explanation: A real and inverted image is formed on the retina, which sends visual information to the brain.

Q3.

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
Explanation: 25 cm is the minimum distance at which an object can be seen clearly without eye strain.

Q4.

The change in focal length of an eye lens is caused by the action of —
(a) pupil (b) retina (c) ciliary muscles (d) iris

✅ Answer: (c) Ciliary muscles
Explanation: These muscles contract or relax to change the curvature of the lens.

Q5.

A person needs a lens of power −5.5 D for distant vision and +1.5 D for near vision.
Find the focal length of each lens.

✅ Answer:
(i) For distant vision P = −5.5 D → f = 1/P = −0.1818 m = −18.18 cm
(ii) For near vision P = +1.5 D → f = 1/P = 0.6667 m = 66.67 cm

Q6.

The far point of a myopic person is 80 cm. Find the nature and power of the correcting lens.

✅ Answer:
f = −0.8 m P = 1/f = −1.25 D
Nature: Concave lens Power: −1.25 D

Q7.

A hypermetropic person’s near point is 1 m. What lens is needed to correct the defect if the normal near point is 25 cm?

✅ Answer:
u = −0.25 m v = −1 m
1/f = 1/v − 1/u = −1 + 4 = 3 → f = 0.333 m
P = 1/f = +3 D
Nature: Convex lens Power: +3 D

Q8.

Why can’t a normal eye see objects clearly closer than 25 cm?

✅ Answer:
Because ciliary muscles cannot contract enough to decrease the focal length further; the image can’t form on the retina.

Q9.

What happens to the image distance in the eye when the object moves farther away?

✅ Answer:
The image distance stays the same (always on the retina); the focal length of the eye lens adjusts accordingly.

Q10.

Why do stars twinkle?

✅ Answer:
Due to atmospheric refraction. Light from stars bends through different air layers of varying densities, making them appear to change brightness and position.

Q11.

Why do planets not twinkle?

✅ Answer:
Planets are closer and appear larger; light from many points averages out the refraction effect, so they shine steadily.

Q12.

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

✅ Answer:
In space, there is no atmosphere to scatter sunlight, so the sky looks dark or black.

❓ Frequently Asked Questions (FAQs)

Q1. What is the least distance of distinct vision for a normal human eye?

Answer: The least distance of distinct vision for a normal human eye is about 25 cm. This means a person with normal eyesight can see objects clearly only if they are at least 25 cm away from the eyes.

Q2. What causes the twinkling of stars?

Answer: Stars twinkle because of atmospheric refraction. The light from stars bends while passing through layers of air of different densities, making their apparent position change continuously.

Q3. Why do planets not twinkle like stars?

Answer: Planets are much closer to Earth and appear larger. The light coming from them averages out the effects of atmospheric refraction, so planets do not twinkle.

Q4. What are the common defects of vision?

Answer: The three main defects of vision are:

Myopia (near-sightedness) – difficulty seeing distant objects, corrected with a concave lens.
Hypermetropia (far-sightedness) – difficulty seeing nearby objects, corrected with a convex lens.
Presbyopia – common in older age due to loss of eye lens flexibility.

Q5. Why does the sky appear dark to an astronaut?

Answer: The sky appears dark or black to an astronaut because space has no atmosphere to scatter sunlight. Scattering of sunlight by the atmosphere causes the sky to look blue on Earth.