PhysicsA concave mirror of a radius 30 cm is placed in water. Its focal length in air and water differ by

A concave mirror of a radius $30 cm$ is placed in water. Its focal length in air and water differ by

A
$15 cm$
B
$20 cm$
C
$30 cm$
D
$0 cm$

Solution:

Its focal length in air and water differ by

0 cm

.
The focal length of a spherical mirror does not depend on the surrounding medium.
It does not change with a medium, nor does it depend on the refractive index of a medium. Hence, the focal length of the mirror in the air is equal to the focal length of the mirror in the water.