Question

A concave mirror and a convex lens are held in water. What changes, if any, do you expect in their focal length?

Answer 1

The focal length of a mirror does not depend upon the nature of the medium in which it is placed, because both the incident as well as reflected ray passes through the same medium. Whereas the focal length of a lens depends upon the medium in which it is placed because it has to go from one medium to another and the degree of refraction varies with the refractive index of the medium.

Thus, there will be no change in the focal length of the concave mirror. Whereas, the focal length of the convex lens will change. If the refractive index of the material of the convex lens is greater than water it will increase; if it is equal to water it will become infinity; if it is less than water it will be negative.