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. (a)`10 cm`
B. (b)`18 cm`
C. (c)`-18 cm`
D. (d)`25 cm`

Answer

C

Explanation

Distance of object from mirror = `15 + (33.25)/(1.33) = 40 cm`
Distance of image from mirror = `15 + (25)/(1.33) = 33.8 cm`
For the mirror , `(1)/(v) + (1)/(u) = (1)/(f)`
`therefore (1)/(-33.8) + (1)/(-40) = (1)/(f) therefore f = -18.3 cm`
`therefore` Most suitable answer is (c)