NCERT Solutions for class 10 Science Chapter 11:Human Eye and Colourful World

NCERT Solutions for class 10 Science Chapter 11 Exercise Question

Question 1

The human eye can focus 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

Question 2

The human eye forms an image of an object at its

(a) cornea

(b) iris

(c) pupil

(d) retina

Answer –

(d) retina

Question 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

Question 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

Question 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

We know Power of lens (P) =

(i) Power of the lens for correcting distant vision = – 5.5 D

So
f = = = -0.181 m
ii)

The focal length is – 0.181 m.

(ii) Power of the lens for correcting near vision = +1.5 D

We know Power of lens (P) = f = = +0.667 m

The focal length is 0.667 m.

Question 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

When the image is formed in front of the retina then person is suffered from myopia.So concave lens is used to correct this defect of eyes.

Let object distance is infinity

Image distance v = – 80 cm

Focal length = f

Using lens formula,

= -
= -
= - 0
=
f =-80 m
We also know
p =
p = -1.25 D

power of concave lens is – 1.25 D

Question 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

When we see distinct objects clearly but difficulty to see nearby object due which image form beyond the retina of eyes then the person suffer from hypermetropia.

Given

Object distance, u = – 25 cm

Image distance, v = – 1 m = – 100 m

Focal length f

Using the lens formula,

= -
= -
= +
= -
=
f =
f =33.3cm = 0.33m We also know
p =
p = 3 D

Power of convex lens +3.0 D.

Question 8

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

Answer

This is due to ciliary muscles of the eyes are unable to contract beyond a certain limit.

Question 9

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

Answer

Due to accommodation our eyes can increase or decrease the focal length so that the image is formed on the retina .So there is no effect on image even on increasing the distance of an object from the eye.

Question 10

Why do stars twinkle?

Answer

The twinkling of a star is due to atmospheric refraction of starlight. The starlight, on entering the earth’s atmosphere, and reaching our eyes change continuously and stars appear to twinkle.

Question 11

Explain why the planets do not twinkle?

Answer

The planets are much closer to the earth, and a collection of a large number of point-sized sources of light, the total variation in the amount of light entering our eye from more than one point sources will average out to zero, thereby nullifying the twinkling effect

Question 12

Why does the Sun appear reddish early in the morning?

Answer

Light of sun has to travel more distance in the atmosphere because it is very far from earth. All colour of lights get scatter except red colour due to longer wavelengths Therefore, the sun appears reddish at sunrise and sunset.

Question 13

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

Answer

Due to absence of atmosphere in space,scattering of light does not take place and sky appear dark instead of blue to an astronaut.