Question

If a concave mirror is dipped in water .will there be any change in focal length

Answer 1

No the focal length will not change in case of mirror but change in case of lens.
As f=R/2 for mirror so in dipping in water focal length for concave mirror will remain constant.

Answer 2

when immersed in water,the difference between light's speed in the lens and the surrounding medium is less than the difference when it is in air, and tje focal length of a lens would be greater under water