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

Options

A. convex,+2.25D
B. convex,+0.25D
C. convex,+0.2D
D. convex,+0.15D

Answer

B

Explanation

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