Brewster's angle

Brewster's angle: For a plane electromagnetic wavefront incident on a plane boundary between two dielectric media having different refractive indices, the angle of incidence at which transmittance from one medium to the other is unity when the wavefront is linearly polarized with its electric vector parallel to the plane of incidence.

For a randomly polarized ray incident at Brewster's angle, the reflected and refracted rays are at 90° with respect to one another.

Source: from Federal Standard 1037C

Brewster's angle is an optical phenomenon first discovered by Sir David Brewster (1781-1868), Scottish physicist.

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:

α_B = arctan( n₂ / n₁ ) ,

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.