A container is filled with water `(mu=1.33)` upto a height of 33.25 cm. A concave mirror is placed 15cm above the water level and the image of an object placed at the bottom is formed 25 cm below the water level. Focal length of the mirror is

Options

A. `10`
B. `15`
C. `20`
D. `25`

Answer

C

Explanation

Image of O is formed at `O^'`
`PO^'=(33.25)/(1.33)=25cm`
Distance of `O^'` from mirror `=25+15=40cm`
Image of `O^'` is formed on `O^'` due to concave mirror hence `O^'` is centre of curvature of mirror
`P^'O^'=40cm=R`
Focal length of mirror: `f=R/2=20cm`