# NCERT Solutions for Class 10 Science Chapter 11 The Human Eye and the Colourful World

NCERT Solutions for Class 10 Science Chapter 11 The Human Eye and the Colourful World are provided in a systematic way which will be useful in making your concepts more strong. These solutions are a comprehensive set of solutions to the problems present in NCERT textbooks.

By using Chapter 11 Class 10 Science NCERT Solutions, students will be able to get an accurate gauge of their understanding of the concepts taught in class. These solutions are available for free online on Gkrankers, making them accessible to everyone.

## Chapter 11 The Human Eye and the Colourful World Class 10 Science NCERT Solutions

### In Text Questions

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

Solution

The ability of an eye lens to change its focal length due to ciliary muscle action is called accommodation. The distance from which a normal eye can see clearly is called the near point.

2. A 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?

Solution

The lens should be concave.
Focal length of concave lens,
f = (for point of defective eye) f = -1.2 m
M = -120 cm

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

Solution

For normal eye, near point is at 25 cm and far point is at infinity.

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

Solution

He is suffering from myopia or short sightedness. It can be corrected by using a concave lens of suitable focal length.

### Exercises

1. The human eye can focus object at different distances by adjusting the focal length of the eye lens. This is due to
(a) presbyopia
(c) near-sightedness
(b) accommodation
(d) far-sightedness

Solution

(b) accommodation

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

Solution

(c) 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

Solution

(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

Solution

(c) ciliary muscles

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

Solution

(i) For distant vision, f1 = 1/P1 = 1/-5.5 = 0.182 cm
or, f1 = -18.2 cm

(ii) For near vision, f2 = 1/P2 = 1/+1.5 = +0.667 cm
or f2 = +66.7 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?

Solution

Given, far point, x = 80 cm
So, using formula f= -x
⇒ f=-80 cm (concave lens)
But, P = 1/f
⇒ P = 1/-0.8
⇒ P = -1.25 dioptre
The required lens is concave lens of power -1.25 D.

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.

Solution

u = -25 cm, v = -1 m = -100 cm
1/f = 1/v – 1/u = 1/-100 – 1/(-25) = -1/100 + 1/25
1/f = -1+4/100 = 3/100 or f = 100/3 cm = 1/3 m
P = 1/f = 1/1/3 = 3
∴ Power = +3 D.
The lens required is convex lens of power +3 D.

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

Solution

For seeing near objects, the ciliary muscles contract to make the lens thicker near middle so as to reduce the focal length of the eye lens. But ciliary muscles cannot be contracted beyond certain limit and hence we cannot see clearly the objects 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?

Solution

Image distance (v) remains the same. When the object is moved, the focal length of eye lens changes due to the action of ciliary muscles so as to satisfy the relation 1/f = 1/v – 1/u in the both the cases.

10. Why do stars twinkle?

Solution

When light travels from one medium to another, it undergoes refraction, due to which its path changes. In our atmosphere there are various layers of air which are having different densities, due to which their refractive indices are also different, i.e. as we came towards the surface of the carts from the atmosphere refractive index increases. When a ray of light enters the atmosphere, it undergoes multiple refractions, due to which apparent position of star keeps on changing as a result, intensity of light coming from stars also fluctuate with time and a star appears to be twinkling.

11. Explain why the planets do not twinkle.

Solution

As planets are much closer to the earth than stars, they can be viewed as a series of point sources. Since the variation in light entering our eyes from these point sources is zero, the twinkling effect is not observed.

12. Why does the sun appear reddish early in the morning?

Solution

In the morning, the sun is near the horizon and the light from the sun passes through thick layers of air and a larger distance in the earth’s atmosphere, scattering blue colour away. Only yellow and red colour reaches our eyes.

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

Solution

It is only up to a certain height that there is an atmosphere. Above that height, however, there is no atmosphere to scatter light. Because there is no atmosphere to scatter light, astronauts perceive the sky as perfectly black.