What is the focal length of a plane mirror?

The focal length of a plane mirror is infinity.

Explanation:

• A plane mirror is a flat surface, so it does not converge or diverge light rays.
• The radius of curvature of a plane mirror is considered infinite.
• Since focal length (f) is related to radius of curvature (R) by the formula:

f = R / 2

If R = infinity, then f = infinity.

Therefore, a plane mirror has an infinite focal length because it does not focus light to a single point.