The focal length of a plane mirror is infinity.
Before letting know about the focal length of plane mirrors we will know what are plane mirrors?
Plane mirrors – The images formed from plane mirrors are the reflected images in their normal proportions but reversed from left to right. These are the most widely used mirrors.
The focal length of a plane mirror is infinity as the image can be formed at an infinite distance inside the mirror. A plane mirror can be recognised as a spherical mirror of infinite radius of curvature. Focal length is half of the radius of curvature and hence it is infinity.