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 instead of a liquid). The electric analog is the Kerr effect.
It was discovered in 1905 by Aimé Cotton and Henri Mouton, working in collaboration and publishing in .
When a linearly polarized wave propagates perpendicularly to a magnetic field (e.g. in a magnetized plasma), it can become elliptized. 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 EX and EO increases.