What is the focal length of a plane mirror?

The focal length of a plane mirror is considered to be infinite. In geometric optics, the focal length is the distance from the focal point to the mirror or lens. For a plane mirror, which is a flat, reflective surface, the reflected rays appear to diverge as if they are coming from a point behind the mirror. However, these rays do not actually converge or diverge to a focal point.

In simpler terms, when parallel rays of light strike a plane mirror, they are reflected in such a way that they appear to be coming from a point behind the mirror. This virtual point is often referred to as the "virtual focus" or "virtual focal point." The key point is that, unlike concave or convex mirrors, a plane mirror does not have a real focal point where the reflected rays converge.

So, for practical purposes and in the context of plane mirrors, the focal length is considered to be infinite because the reflected rays never actually meet at a specific point. The image formed by a plane mirror is virtual, upright, and laterally inverted, and it appears to be as far behind the mirror as the object is in front of it.