what is the focal length of a plane mirror? please explain

Focul length is the geometric property of optical systems to focus or defocus i.e. diverge the incoming rays.

It's related to the radii of curvature of the surfaces of the optical system or component.

In a plane mirror, the radius of curvature is infinity. So incomes rays evidently don't converge or diverge.

But the focal length or the point of focus or defocus must be very very far away.

It's infinity.

Strictly speaking, f = -R/2

where f is the focal length and R is the radius of curvature of the spherical mirror.

R = inifinity for plane mirror, and hence f.

Regards,

Team,

TopperLearning.