Parabolic Mirrors:

A parabolic mirror is not the same as a circular mirror.  Fist, the focal point depends on the equation of the mirror.
If the equation is
y = a*x^2
where the origon is where the mirror crosses the surface, then the focal length is
f = 1/(4*a)

(Bauer and Westfall, pg. 1040)

Parabolic mirrors can be converging or diverging based on which side is reflecting:
the inside is converging, the outside is diverging.

Example:


(DuraVision Mirror Systems)

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