Page 175 - NCERT Science Class 10 English Medium

What you have learnt

n The ability of the eye to focus on both near and distant objects, by adjusting its
focal length, is called the accommodation of the eye.
n The smallest distance, at which the eye can see objects clearly without strain, is
called the near point of the eye or the least distance of distinct vision. For a young
adult with normal vision, it is about 25 cm.
n The common refractive defects of vision include myopia, hypermetropia and
presbyopia. Myopia (short-sightedness – the image of distant objects is focussed
before the retina) is corrected by using a concave lens of suitable power.
Hypermetropia (far-sightedness – the image of nearby objects is focussed beyond
the retina) is corrected by using a convex lens of suitable power. The eye loses its
power of accommodation at old age.
n The splitting of white light into its component colours is called dispersion.
n Scattering of light causes the blue colour of sky.

E X E R C I S E S

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.
2. The human eye forms the image of an object at its
(a) cornea. (b) iris. (c) pupil. (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.
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.
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?
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?
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.
8. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
9. What happens to the image distance in the eye when we increase the distance of an
object from the eye?
10. Why do stars twinkle?
11. Explain why the planets do not twinkle.
12. Why does the sky appear dark instead of blue to an astronaut?

170 Science

2024-25