Reflectance of thin Ulms

The percentage of light reflected at an interface between two materials (reflectance) depends on the refractive index and the angle of incidence.

For an uncoated piece of crown glass (refractive index = 1.52),

For the same glass coated with a material of index nc


2 1

J ngl

ass nc j

1 ngl'<

This gives the interesting result that R = 0 when nc =

There are no suitable materials with this refractive index. Magnesium fluoride (nc = 1.38) is cheap and durable and widely used. For a single coating of MgF2, R = 1.3%

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