A man, wearing glasses of power `+2D` can read clearly a book placed at a distance of `40cm `from the eye. The power of the lens required so that he can read at `25cm` from the eye is

Options

A. `+4.5D`
B. `+ 4.0D`
C. `3.5D`
D. `+ 3.0D`

Answer

C

Explanation

`(1)/(-(NP)) - (1)/(-(distance of object))= (1)/(f)`
`(1)/(-(N.P)) - (1)/(-40) = (1)/(100//2)`
` = (1)/(-N.P) = (1)/(50) - (1)/(40) = (-1)/(200)`
`rArr " " N.P = 200 cm`
` = (1)/(-N.P) - (1)/(-25) = (1)/(f)`
` (1)/(-200) +(1)/(25) = (7)/(200) = (1)/(f) rArr f = (200)/(7) cm`
` P = (100)/(f(cm))) = (100)/(200//7) = 3.5`