Question

A person can see clearly objects only when they lie between 50 cm and 400 cm from his eyes. In order to increase the miximum distance of distinct vision to infinity , the person has to use, will be
A. convex, + 2.25 D
B. concave, - 0.25 D
C. concave, - 0.2 D
D. convex, + 0.15 D

Answer 1

Correct Answer - B
Image distance, `v=400 cm = 4 m rArr u = oo`
Using lens equation,
`(1)/(v)-(1)/(u)=(1)/(f)`
`rArr " " (1)/(-4)-(1)/(oo)=(1)/(f)rArr f=-4m`
Now, power of the required lens is `P=(1)/(f)=(1)/(-4)=-0.25 D`
Thus, the person require a concave lens of power `-0.25 D`.