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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 , the Voigt effect is proportional to the square of the magnetization and can be seen experimentally at normal incidence. There are several denominations for this effect in the literature: the Cotton–Mouton effect , the Voigt effect , 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 E i → = e − i ω {\displaystyle {\vec {E_{i}}}={\begin{pmatrix}\cos \beta \\\sin \beta \\0\end{pmatrix}}e^{-i\omega }} and an in-plane polarized sample m → = {\displaystyle {\vec {m}}={\begin{pmatrix}\cos \phi \\\sin \phi \\0\end{pmatrix}}} , the expression of the rotation in reflection geometry is δ β {\displaystyle \delta \beta } is :
and in the transmission geometry :