What is the focal length of a plane mirror?
Answer
The focal length of a plane mirror is infinity, because its reflecting surface is flat and does not converge or diverge parallel rays to a point.
The working
Key result
A plane mirror has an infinite focal length, and therefore its principal focus lies at infinity.
Reason
Parallel rays striking a plane mirror reflect as parallel rays and never meet, so no real focal point is formed at a finite distance.
Link to radius
Focal length is half the radius of curvature (f = R ÷ 2). A plane mirror is like a spherical mirror of infinite radius, so f is infinite.
Power
Since power is the reciprocal of focal length, a plane mirror has zero power because its focal length is infinite.
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