Question

what is the focal length of plane mirror

Answer 1

In case of convex and concave mirrors (i e spherical mirrors) , the centre of curvature is the centre of the sphere of which the mirror forms a part and the radius of curvature is the radius of the sphere of which the mirror forms a part. The focal length is half of the radius of curvature. Now , let us imagine plane mirror to be a part of a sphere with infinite radius. As a result, the focal length will be infinity. Hence, we may say that the focal length exists only for spherical mirrors and not plane mirrors.