Class 10 Science · Chapter 10

10. The Human Eye and the Colourful World

Complete solutions to the intext questions and exercises. CBSE Class 10 Science (NCERT).

Intext Questions (Page 164)

1What is meant by power of accommodation of the eye?

The ability of the eye lens to adjust its focal length is called the power of accommodation. The ciliary muscles change the curvature of the eye lens. This helps us to see both nearby and distant objects clearly.

2A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?

The person should use a concave lens (diverging lens) of suitable power.

Its focal length should be equal to the far point, i.e. f = −1.2 m. So its power is P = 1/(−1.2) = −0.83 D.

3What is the far point and near point of the human eye with normal vision?

Far point: Infinity

Near point: 25 cm from the eye

4A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?

The child is suffering from myopia (near-sightedness). The child can see nearby objects clearly but not distant objects.

It can be corrected by using spectacles with a concave lens of suitable power. The concave lens brings the image back on to the retina.

Exercises

1The 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.

2The human eye forms the image of an object at its (a) cornea. (b) iris. (c) pupil. (d) retina.

Answer: (d) retina.

3The 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.

4The 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.

5A 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?

(i) Distant vision (P = −5.5 D)

f = 1/P = 1/(−5.5) = −0.182 m = −18.2 cm (concave lens)

(ii) Near vision (P = +1.5 D)

f = 1/P = 1/1.5 = 0.667 m = +66.7 cm (convex lens)

6The 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?

A concave lens is needed. It should form the image of a far object (at infinity) at the far point.

u = −∞, v = −80 cm
1/f = 1/v − 1/u = 1/(−80) − 0, so f = −80 cm = −0.8 m
P = 1/f = 1/(−0.8) = −1.25 D

7Make 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.

(a) Hypermetropic eye: the image of a near object is formed behind the retina. (b) Correction with a convex lens.

A convex lens is used. It should form the image of an object at 25 cm at the near point of the eye (100 cm).

u = −25 cm, v = −100 cm
1/f = 1/v − 1/u = −1/100 + 1/25 = (−1 + 4)/100 = 3/100
f = 100/3 cm = 33.3 cm = 0.333 m
P = 1/f = 1/0.333 = +3 D

A convex lens of power +3 D is required.

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

The focal length of the eye lens cannot be decreased below a certain limit. So the eye lens cannot focus the image of an object closer than 25 cm on the retina. The object looks blurred and the eye gets strained.

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

The image distance in the eye does not change. The image is always formed on the retina. When the object moves away, the ciliary muscles relax. The eye lens becomes thinner and its focal length increases. So the image still forms on the retina.

10Why do stars twinkle?

Light from a star passes through the atmosphere before reaching us. The atmosphere has layers with different refractive indices, and they keep changing. So the light from the star refracts again and again. The apparent position of the star changes slightly. The amount of light entering our eye also changes. So the star looks sometimes brighter and sometimes fainter. This is twinkling.

11Explain why the planets do not twinkle.

Planets are much closer to the earth than stars. So a planet acts like an extended source, a collection of many point sources. The changes in light from all these points cancel each other. So the average brightness stays the same, and planets do not twinkle.

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

The sky looks blue because of scattering of light by particles in the atmosphere. At very high altitudes there is almost no atmosphere. So there is no scattering of light. That is why the sky appears dark to an astronaut.

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