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

Answer

A

Explanation

`1/f = (n/1 - 1) (1/(R_1) - 1/(R_2))`
`1/(f_1) - (n/(n.) - 1)(1/(R_1) - 1/(R_2))`
`(f_1)/(f) = ((n - 1)n.)/(n - n.) " or " f_1 = (-fn.(n- 1))/(n. - n)`