A container is filled with water `(mu = 1.33)` up to height of `33.2 cm`. A concave mirror is placed 15 cm above the water level. Image of an object placed at the bottom is formed at the same place. The focal length of mirror is :

Options

A. 10 cm
B. 15 cm
C. -20 cm
D. 2 cm

Answer

C

Explanation

Apparent depth of water can be written as = 33.2/1.33 = 24.962 = 25 cm
hence object is effectively at a distance 25 + 15 = 40 cm from the concave mirror. Final image is formed at the same place and hence light rays from object must be falling normally on the mirror . So 40 cm must be radius of curvature of mirror and hence 20 cm would be its focal length . Since it is concave mirror and hence focal length is - 20 cm.