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

Answer

C

Explanation

`(1)/(-(N.P.)) - (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. = 200cm`
`(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) = 3.5D`