JEE Physics 2025 Question with Solution

Given: A concave mirror has focal length $f$ in air and is dipped in a liquid of refractive index $\mu$.

Find: The focal length of the mirror in the liquid.

A mirror forms images by reflection, not by refraction. Therefore, its focal length depends only on its geometry, that is, the radius of curvature.

Using the mirror relation:

Here, $R$ is the radius of curvature of the mirror. Since the refractive index of the surrounding medium does not appear in this expression, changing the medium from air to a liquid does not change the focal length.

Therefore, the focal length remains $f$ in the liquid.

The correct option is D.

Given: A concave mirror is immersed in a liquid of refractive index $\mu$.

Find: Whether its focal length changes.

For mirrors, reflection is the governing phenomenon, so the surrounding medium does not affect focal length. Only lenses change focal length when placed in another medium because refraction depends on refractive index.

Hence, the focal length stays $f$.

The correct option is D.