Focal Length of a Plane Mirror
A plane mirror does not have a focal length.

Explanation:
- A plane mirror is a flat, smooth surface that reflects light without distorting the image.
- When light rays strike a plane mirror, they are reflected at the same angle at which they hit the mirror.
- The reflected rays appear to come from a point behind the mirror, which is known as the virtual image.
- Since the reflected rays do not actually converge at a focal point, a plane mirror does not have a focal length.
- The distance between the object and its virtual image in a plane mirror is equal to the distance between the object and the mirror.
- This characteristic of a plane mirror makes it useful in applications such as dressing mirrors, periscopes, and optical instruments.
In conclusion, a plane mirror does not have a focal length because the reflected rays do not converge at a focal point. Instead, they form a virtual image that appears to be the same distance behind the mirror as the object is in front of it.