The focal length of a concave mirror is f in air. Its focal length in water (refractive index ) will be

CONCEPT:

Concave mirror: 

• If the inner surface of the spherical mirror is the reflecting surface, then it is called a concave mirror. It is also called a focusing mirror/converging mirror.
• • The size of the image produced by these mirrors can be larger or smaller than the object, depending upon the distance of the object from the mirror.
  • The concave mirror can form both real as well as virtual images of any object.
  • The concave mirror converges ray of lights into a point called focus.
  • The distance between the pole (Center of reflecting surface) and the focus of a concave mirror is termed as focal length (F).  

EXPLANATION:

• In case of concave mirrors, there will be no change in the focal length of the mirrors when immersed in water. Therefore the focal length of concave mirror is 'f'.
• It is so because the focal length of mirrors does not depend on the external medium in which it is held.