Practice portal › Ray Optics and Optical Instruments › Reflection by Plane and Spherical Mirrors
Asked in GSEB Board March 2023 · Plane mirrors
Given: a plane mirror.
Idea: the power of a mirror measures how strongly it changes the convergence of a beam, and is the reciprocal of its focal length in metres.
A plane mirror is the limiting case of a spherical mirror whose radius of curvature has grown without bound, so f = R/2 is infinite as well.
P = 1/f = 0.
Physically this says the mirror changes the direction of light but not its convergence: parallel rays come back parallel, and a point object gives a point image the same size as the object.
So the power of a plane mirror is 0.
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