material dielectric constant, the Voigt parameter, and two cubic constants describing magneto-optical effect depending on . is the reduce . Calculation is made in the spherical approximation with . At the present moment, there is no evidence that this approximation is not valid, as the observation of Voigt effect is rare because it is extremely small with respect to the Kerr effect. Eigenvalues and eigenvectors To calculate the eigenvalues and eigenvectors, we consider the propagation equation derived from the Maxwell equations, with the convention : When the magnetization is perpendicular to the propagation wavevector, on the contrary to the Kerr effect, may have all his three components equals to zero making calculations rather more complicated and making Fresnels equations no longer valid. A way to simplify the problem consists to use the electric field displacement vector . Since and we have . The inconvenient is to deal with the inverse dielectric tensor which may be complicated to handle. Here calculation is made in the general case which is complicated mathematically to handle, however one can follow easily the demonstration by considering . Eigenvalues and eigenvectors are found by solving the propagation equation on which gives the following system of equation: where represents the inverse element of the dielectric tensor , and . After a straightforward calculation of the system's determinant, one has to make a developpement on 2nd order in and first order of . This led to the two eigenvalues corresponding the two refraction indices: The corresponding eigenvectors for and for are: Reflection geometry Continuity relation Knowing the eigenvectors and eigenvalues inside the material, one have to calculate the reflected electromagnetic vector usually detected in experiments. We use the continuity equations for and where is the induction defined from Maxwell's equations by . Inside the medium, the electromagnetic field is decomposed on the derived eigenvectors . The system of equation to solve is: The solution of this system of equation are: Calculation of rotation angle The rotation angle and the ellipticity angle are defined from the ratio with the two following formulae: where and represent the real and imaginary part of . Using the two previously calculated components, one obtains: This gives for the Voigt rotation: which can also be rewritten in the case of , , and real: where is the difference of refraction indices. Consequently, one obtains something proportional to and which depends on the incident linear