The focal length of a plano-concave lens is -10 cm , then its focal length when its palne surface is polished is

Options

A. `20 cm`
B. `-5 cm`
C. `5 cm`
D. none of these

Answer

D

Explanation

`(1)/(f)=(1)/(infty)-(2)/(f_(l))=-(2)/(f_(l))=(1)/(-28)`
`f_(l)=56 cm rArr (mu-1)((1)/(R))=(1)/(56)` .............`(i)`
`((14)/(9)-1)(1)/(R)=(1)/(58)=(280)/(9) cm`