Question

what is a focal length of plane mirror​

Answer 1

Infinity is the focal length of the plane mirror ✔

Answer 2

here your answer

(A plane mirror can be considered as a spherical mirror of infinite radius of curvature. Focal length is half of radius of curvature and hence it is infinity. You can use the mirror formula also to get the focal length of a plane mirror, where the image distance = − object distance.)