Practice portal › Ray Optics and Optical Instruments › Reflection by Plane and Spherical Mirrors
Asked in GUJCET 2015 · Plane mirrors
Idea: a plane mirror is the limiting case of a spherical mirror whose radius of curvature has grown infinite.
With R=∞ the focal length is f=R/2=∞.
Power is the reciprocal of the focal length in metres, P=1/f.
So P=1/∞=0.
This matches what a plane mirror does: it neither converges nor diverges a beam, so it has no power.
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