Question

The reciprocal of focal length of the lens is called power of lens. The power of the lens can be measured in Dioptre. The focal length of eye lens of eye lens of eye of Jaishri is 25cm. What is the power of the eye lens of eye Jaishree? Which type of the lens should be used in the spectacles of her.

a. +4.0D, convex lens
b. +4.0D, concave lens
c. -4.0D, convex lens
d. -4.0D, concave lens

Answer 1

Option (a) is correct +4.0 D is the correct answer with convex lens.

Answer 2

Answer:

A negative concave lens must be used for correction.

Explanation:

Given the focal length of eye lens, $f = 25cm=0.25m$

The reciprocal of focal length of the lens measured in meters is called power of lens.

$P=\dfrac{1}{f}$

The power of the lens can be measured in Dioptre.

Therefore, the power of the eye lens is  

$P=\dfrac{1}{0.25}=4D$    

The near point of a human eye defined as the shortest object distance that a normal adult eye is able to accommodate, or to image onto the retina is 25cm.

So, the problem here is the focusing of the distant objects, the far point is no longer at infinity.

Hence, a negative lens i.e., concave lens must be used for correction.

So, option d is the correct answer.