Answer:
The surface of a plane mirror is flat.
Step-by-step explanation:
A plane mirror is a part of a curved mirror whose radius of curvature is very very large. the radius of curvature of such curved mirror is approx. infinity.
The relation between the radius of curvature and the focal length is given by
As, R is approx infinity , so the focal length is also infinity.
Thus, the focal length of a plane mirror is approx infinity.