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. Commonly in literature, one can find several denomination for this effect such as Cotton-Moutton effect (in reference to French scientists Aimé Cotton and Henri Mouton), the Voigt effect (in reference to the German scientist Woldemar Voigt) or also as magnetic-linear birefringence. This last denomination is closer from 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.