The Voigt effect is a magneto-optical phenomenon which rotates and elliptizes linearly polarised light sent into an optically active medium. Unlike many other magneto-optical effects such as the Kerr or Faraday effect which are linearly proportional to the magnetization (or to the applied magnetic field for a non magnetized material), the Voigt effect is proportional to the square of the magnetization (or square of the magnetic field) and can be seen experimentally at normal incidence. There are several denominations for this effect in the literature: the Cotton–Mouton effect (in reference to French scientists Aimé Cotton and Henri Mouton), the Voigt effect (in reference to the German scientist Woldemar Voigt), and magnetic-linear birefringence. This last denomination is closer in the physical sense, where the Voigt effect is a magnetic birefringence of the material with an index of refraction parallel () and perpendicular ) to the magnetization vector or to the applied magnetic field. For an electromagnetic incident wave linearly polarized and an in-plane polarized sample , the expression of the rotation in reflection geometry is is: and in the transmission geometry: where is the difference of refraction indices depending on the Voigt parameter (same as for the Kerr effect), the material refraction indices and the parameter responsible of the Voigt effect and so proportional to the or in the case of a paramagnetic material. Detailed calculation and an illustration are given in sections below. Theory As with the other magneto-optical effects, the theory is developed in a standard way with the use of an effective dielectric tensor from which one calculates systems eigenvalues and eigenvectors. As usual, from this tensor, magneto-optical phenomena are described mainly by the off-diagonal elements. Here, one considers an incident polarisation propagating in the z direction: the electric field and a homogenously in-plane magnetized sample where is counted from the [100] crystallographic direction. The aim is to calculate where is the rotation of polarization due to the coupling of the light with the magnetization. Let us notice that is experimentally a small quantity of the order of mrad. is the reduced magnetization vector defined by , the magnetization at saturation. We emphasized with the fact that it is because the light propagation vector is perpendicular to the magnetization plane that it is possible to see the Voigt effect. Dielectric tensor Following the notation of Hubert, the generalized dielectric cubic tensor take the following form: where is the