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Focusing surfaces
Published online by Cambridge University Press: 01 August 2016
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In standard textbooks (e.g. [1, p. 877]) as well as in Encylopaedia Britannica [2, Vol. 13, p. 611], one finds variants of the following statement:
In spherical mirrors, rays parallel and very close to the principal axis are all reflected through a single point.
This statement is actually false, and can only be understood in the following sense: It is true only up to a linear approximation, and in fact, as soon as a mirror focuses parallel rays, it must be a parabolic mirror.
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