Faraday rotator

Thus it is an example of circular birefringence, as is optical activity, but involves a material only having this property in the presence of a magnetic field.

The polarization state is rotated in proportion to the applied longitudinal magnetic field according to: where

This empirical proportionality constant (in units of radians per tesla per metre, rad/(T·m)) varies with wavelength and temperature[1][2][3] and is tabulated for various materials.

Unlike the rotation in an optically active medium such as a sugar solution, reflecting a polarized beam back through the same Faraday rotator does not undo the polarization change the beam underwent in its forward pass through the medium, but actually doubles it.

In an optically active medium, the polarization direction twists or rotates in the same sense (e.g. like a right-handed screw) for either direction, thus in the case of a plane reflection the original rotation is reversed, returning the incident beam to its original polarization.