Question

A concave mirror of focal length f in vscuum is placed in a medium of refractive index 2. Its focal length in the nedium is

Answer 1

Mirrors, spherical or otherwise, operate on the principle that the angle of reflection of a ray of light equals the angle at which it strikes the mirror's surface. That behavior is not affected by the medium in which the light travels, so the mirror's focal length would be no different in air or water.