Q: A person needs a lens of power −5.5 dioptres for correcting his distance 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) distance vision, and (ii) near vision?

(i) Distant Vision :Given, Power =5.5D Power, P=f1 Focal length, f=P1 ⇒f=−5.51 ⇒f=−5510 ⇒f=−112 ⇒f=−0.181m ⇒f=−18.1cm(ii) Near Vision :Given,Power =1.5D P=f1 f=P1 ⇒f=1.51 ⇒f=1510 ⇒f=32 ⇒f=0.667m ⇒f=66.7cm

Question 1

The human eye can focus objects at different distances by adjusting the focal length of the eye.This is due to:

Accepted Answer

Due to Accommodation, the human eye can focus objects at different distances.The accommodation of an eye is its reflex to adjust itself in response to focusing on a near object and then suddenly focusing on a faraway object. Option B is correct.

Question 2

Define 1 dioptre power of a lens.

Accepted Answer

1 dioptre is the power of a lens whose focal length is 1 metre.

Question 3

What is meant by power of accommodation of the eye?

Accepted Answer

The accommodation of an eye is its reflex to adjust itself in response to focusing on a near object and then suddenly focusing on a far away object. This accommodation of the eye comprises of coordinated changes in convergence, lens shape, and pupil size.

Question 4

The human eye forms the image of an object at its

Accepted Answer

The image of any object placed in front of the human eye is formed at the retina of the eye which is present as the screen being the eye lens.

Question 5

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.

Accepted Answer

Given that,
The person can't see objects beyond 1.2 m directly
So, in order to view the distances beyond 1.2 m, the focal length of the lens should be 1.2 m.
Now, the power
P=f1​
P=1.21​
P=0.83D
So, the person suffers from Myopia, the corrective lens will be concave or diverging lens.
Hence, the power is 0.833D

Question 6

The least distance of distinct vision for a young adult with normal vision is about

Accepted Answer

Near point or least distinct vision - it is the distance from the eyes up to which eyes can have a clear vision is called least distinct vision. It is about 25cms for a normal healthy eye.

Question 7

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

Accepted Answer

The near point of the eye is the minimum distance of the object from the eye, which can be seen distinctly without strain. For a normal human eye, this distance is 25 cm.
The far point of the eye is the maximum distance to which the eye can see the objects clearly. The far point of the normal human eye is infinity.

Question 8

The change in focal length of an eye lens is caused by the action of the :

Accepted Answer

Answer is C.Ciliary muscle is a circular muscle that relaxes or tightens the zonules to enable the lens to change shape for focusing. The zonules are fibers that hold the lens suspended in position and enable it to change shape during accommodation. It increases or decreases the focal length of the eye lens.

Question 9

A 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?

Accepted Answer

A student has difficulty in reading the blackboard while sitting in the last row. It shows that he is unable to see distant objects clearly. He is suffering from myopia. This defect can be corrected by using a concave lens.

Question 10

A person needs a lens of power −5.5 dioptres for correcting his distance 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) distance vision, and (ii) near vision?

Accepted Answer

(i) Distant Vision :Given,     Power =5.5D    Power, P=f1​    Focal length, f=P1​    ⇒f=−5.51​    ⇒f=−5510​    ⇒f=−112​    ⇒f=−0.181m    ⇒f=−18.1cm(ii) Near Vision :Given,Power =1.5D     P=f1​    f=P1​    ⇒f=1.51​    ⇒f=1510​    ⇒f=32​    ⇒f=0.667m    ⇒f=66.7cm

Question 11

The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of lens required to correct the problem?

Accepted Answer

The person is suffering from myopia and hence need a concave lens to correct the defect. The lens should be such that an object at infinity must form its image at the far point. Hence, f=−80 cm=−0.8 mThe power of the lens can be obtained as:P=f1​P=(−0.8)1​=−1.25 D

Question 12

Make a diagram to show how hypermetropia is corrected. The near point of hypermetropic eye is 1 m. What is the power of lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.

Accepted Answer

Hypermetropia can be corrected by using a convex lens. A convex lens converges the incog light such that the image is formed on the retina.An object at 25 cm forms an image at the near point of the hypermetropic eye. Here, near point is 1 m.Given,u=−25 cmv=−100 cmFrom lens formula, v1​−u1​=f1​ v1​−u1​=f1​                                Focal length,f=100/3 cm=1/3 m−1001​−−251​=f1​

Question 13

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

Accepted Answer

Ciliary muscles contract and expand to change the focal length of eye lens so that it is able to see objects at variable distances. However, too much contraction lays a lot of stress on the eye muscles and when objects are closer than the near point(25 cm), the eye is unable to focus its image on retina. Hence, a normal eye cannot see objects closer than 25 cm clearly.

Question 14

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

Accepted Answer

The  size of the eye can change, so the image distance  is fixed . When we increase the distance of
an object  from  the eye , the image  distance 
in the eye  does  not 
change.  Due to power of accommodation
of the eye, focal  length of the eye lens
is changed which compensates the increase in object  distance. Image distance remains fixed  and image is formed on the retina of the eye.

Question 15

Why do stars twinkle?

Accepted Answer

Light from stars passes through the atmosphere. The different layers of atmosphere are moving hence, its refractive index is changing. As the light is bent at different angles at different points of time, their position does not appear fixed and hence, they twinkle.

Question 16

Explain why the planets do not twinkle.

Accepted Answer

Twinkling occurs because the apparent position of a heavenly body is not fixed because the light through them is refracted through the atmosphere. Planets appear larger to human eyes as compared to stars because they are closer. As the angle subtended by the planets at the human eye is more, the shift in their position(as compared to stars) is smaller. Since, the shift is smaller, they do not twinkle.

Question 17

Why does the Sun appear reddish early in the morning?

Accepted Answer

During sunrise, the light rays from the sun travel a greater distance in the earth's atmosphere. Hence, these light rays are scattered more than they are scattered during noon. Also, red light is scattered the least and hence travel the longest distance. All the other light rays in the visible spectrum is scattered away. Hence, sun appears reddish in the morning.

Question 18

Why does the sky appears dark instead of blue to astronauts ?

Accepted Answer

When sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelength more strongly than red. The scattered blue light enters our eyes. If the earth had no atmosphere, there would not have been any scattering . Then the sky would have looked dark. The sky appears dark to astronaut flying at very high attitude, as scattering is not proent at such heights.

Chapter 11 : The Human Eye and Colourful World