In physical optics, the Cotton–Mouton effect refers to 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.
When a linearly polarized wave propagates perpendicular to magnetic field (e.g. in a magnetized plasma), it can become elliptized. Because a linearly polarized wave is some combination of in-phase X & O modes, and because X & O waves propagate with different phase velocities, this causes elliptization of the emerging beam. As the waves propagate, the phase difference (δ) between EX & EO increases.