Flat Mirror
Optical cross section of a flat mirror with a given reflectivity at a particular wavelength can be express by this formula.
Where is the cross sectional diameter of the beam. Note that the direction of the light has to be perpendicular to the mirror surface for this formula to be valid, else the return from the mirror would no longer go back to it source.
In order to maximize the return a corner reflector is used. The alignment of a corner reflector with respect to the source is not as critical as the alignment of a flat mirror.
Read more about this topic: Optical Cross Section
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