When light moves between two media of differing refractive index, light which is p-polarised with respect to the interface will not be reflected from the interface at one particular incident angle, known as Brewster's angle.
It may be calculated by:
where n1 and n2 are the refractive indices of the two media.
Note that, since all p-polarised light is refracted, any light reflected from the interface at this angle must be s-polarised. A glass plate placed at Brewster's angle in a light beam can thus be used as a polariser.
See also Fresnel equations.