Brewster's angle (also known as Polarization angle) is an optical phenomenon first discovered by the Scottish Physicist, Sir David Brewster (1781-1868).

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:

<math>\theta_B = \arctan \left( \frac{n_2}{n_1} \right) <math>,

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.

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

For a glass medium (n₂≈1.5) in air (n₁≈1), Brewster's angle for visible light is approximately 56°.

See also:

• Fresnel equations
• Snell's law

de:Brewsterwinkel pl:Kąt Brewstera