Question

If a spherical mirror is immersed in a liquid its focal length:
A) Decreases
B) Increases
C) Remains same
D) Increases and decreases

Answer 1

Hint
The phenomenon of reflection is independent of medium. The geometry of the mirror decides the nature of reflection. Mediums have different frequencies. Mirror doesn’t reflect light differently depending on the medium. Focal length of a mirror is a property of the mirror itself. Hence there is no difference in the focal length.

Complete step by step answer
Focal length of a mirror is the distance of a point on the principal axis from the pole of the mirror where, all the light beams travelling parallel to the principal axis of the mirror or the lens coverage. The location of the focal length depends on the curvature of the mirror or lens. For a concave mirror its focal length lies in the front of the mirror and is treated as positive. Whereas for a convex mirror, its focal length is located at the back of the mirror and is treated as negative (light rays do not actually coincide with at this point but they appear to be emerging from this point when traced backwards). Therefore the focal length is a property of the mirror and is independent of the environment.

Option (C) is correct answer.

Note
Focal length is a quantity defined for several optical instruments. And these depend on various factors. One can go wrong if they don’t realise which factor they have to look at. E.g. The refraction by lenses is dependent on the medium. So their focal lengths also vary from medium to medium. For lenses, the case of focal length is the opposite as convex lenses have a positive focal length and concave lenses have a negative focal length.