Practice portal › Ray Optics and Optical Instruments › Reflection by Plane and Spherical Mirrors
Asked in GSEB Board March 2019 · Plane mirrors
Idea: a plane mirror is a spherical mirror whose centre of curvature has receded to infinity.
That is what m=1 is telling us: the image is the same size as the object and as far behind the mirror as the object is in front, for every object distance. No curved mirror behaves like that.
So R=∞.
The focal length is half the radius of curvature, f=R/2.
With R=∞ this gives f=∞ — the mirror brings a parallel beam to no focus at all, which is the same as saying its power is zero.
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