Glossary

Anisotropic medium: medium in which the speed of a light wave and therefore the refraction index depend on the direction of propagation and on the polarization
Birefringence: property of an anisotropic medium in which light propagates differently depending on its polarization
Uniaxial material: said of a birefringent material for which the ordinary and extraordinary refraction indexes are identical for plane waves propagating in a given direction called the optical axis
Birefringent plate: plate made of a birefringent material
Refraction index: for a transparent medium, ratio between the light speed in the air and the light speed in the medium ; for a reflective surface in the air or vacuum, the refraction index is « - 1 »
Optical axis ( birefringent material ): direction of propagation of a wave in a birefringent material for which the polarization remains unchanged
Slow axis: for a birefringent medium and a given plane light wave, polarization direction with the highest refraction index
Fast axis: for a birefringent medium and a given direction of light wave propagation, said of the polarization orientation with the lowest refraction index
Optical path ( or optical length ): product of a length (along the path of a light ray) by the refraction index of the medium