Question

A concave mirror and convex lens are immersed in water. What change, if any, do you expect in the focal length of the two.

Answer 1

Hint The lenses use the phenomenon of refraction to converge or diverge a beam of light. A spherical mirror uses the phenomenon of reflection to converge or diverge a beam of light. The refractive index of water is different from the refractive index of air.

Complete Step by step answer
A concave mirror is a spherical mirror that is polished on the surface which is curved inward. The polished curved surface reflects any light falling on it according to the law of reflection, which states that the angle of incidence should be equal to the angle of reflection. The focus of a concave mirror is situated at a distance which is half of the radius of curvature of the mirror.
When kept inside water, the laws of reflections are still valid, thus the focal length of the concave mirror does not change.
A convex lens is an optical lens, which is curved outwards from one side or both sides. It is known as a converging lens because its shape deviates a beam of light in such a way that the rays get converged after refracting from the convex lens. The refraction of light when it travels from one medium to the other depends on the refractive index of both mediums. We know that the refractive index of air is different than that of water, the converging capacity of a convex lens will change when it is immersed in water.
Since water has a greater value of refractive index than air, the converging power of the lens will be less as compared to air. Thus the focal length of the convex lens will increase.

Note The direction of light changes only when it travels through two mediums of different refractive indexes. Otherwise, the light propagates in a straight line when the medium has a constant value of refractive index. Thus the light will travel in a straight line, be it air or water.