In physical optics, the Cotton–Mouton effect is the birefringence in a liquid in the presence of a constant transverse magnetic field. It is a similar but stronger effect than the Voigt effect (in which the medium is a gas). Its electric analog is the Kerr effect.^[1]

It was discovered in 1905 by Aimé Cotton and Henri Mouton, working in collaboration and publishing in Comptes rendus hebdomadaires des séances de l'Académie des sciences.^[2][3]

When a linearly polarized wave propagates perpendicularly to a magnetic field (e.g. in a magnetized plasma), it can become elliptically polarized. Because a linearly polarized wave is some combination of in-phase X and O modes, and because X and O waves propagate with different phase velocities, there is elliptization of the emerging beam. As the waves propagate, the phase difference (δ) between E_X and E_O increases.

• Cotton effect