The near point of a hypermetropic person is `50 cm` from the eye. What is the power of the lens required to enable him to read clearly a book held at `25 cm` from the eye ?

Explanation

The book placed at 25 cm from the corrective lens should have its virtual image formed at 50 cm from the lens.
Given, object distance, `u=-25cm`
Image distance, `v=-50cm`
From lens formula, focal length of the corrective lens is
`(1)/(f)=(1)/(v)-(1)/(u)=(1)/(50)+(1)/(25)=(1)/(50)`
`rArr" "f=50cm =0.5m`
Power of the lens is
`P=(1)/(f)=(1)/(0.50)=2D`
So, the corrective lens is a convex (converging) lens of power 2 D.