A lens of refractive index n is put in a liquid of refractive index `n'`. If focal length of lens in air is `f`, its focal length in liquid will be.

Options

A. `(fn'(n-1))/(n'-n)`
B. `f(n'-1)/(n'(n-1))`
C. `(n'(n-1))/(f(n'-1))`
D. `(fn'n)/(n-n')`

Answer

A

Explanation

`(1)/(f)=(n-1)((1)/(R_1)-(1)/(R_2))` ..(i)
`(1)/(f_l)=((n)/(n'-1))((1)/(R_1)-(1)/(R_2))` ..(ii)
(i)/(ii)
`f_(l)/(f)=((n-1))/((n/(n')-1))=((n-n)n')/((n-n'))`
`f_l=((n-1)n'f)/((n-n'))=-((n-1)n'f)/((n'-n))`