The focal length of a plane mirror is considered to be infinite. This is because a plane mirror does not converge or diverge light rays; instead, it reflects them in such a way that the outgoing rays remain parallel to the incoming rays.
In geometric optics, the focal length (f) is defined as the distance from the mirror or lens to the focal point, where parallel rays of light either converge (in the case of converging mirrors or lenses) or appear to diverge from (in the case of a diverging lens or mirror). Since a plane mirror does not have a single focal point where the rays converge or diverge, it is said to have an infinite focal length.