Birefringence, or double refraction, is the decomposition of a
ray of
light into two rays (the ordinary ray and the extraordinary ray) when it passes through certain types of material, such as
calcite crystals or
boron nitride, depending on the
polarization of the light. This effect can occur only if the structure of the material is
anisotropic. If the material has a single axis of anisotropy or
optical axis, (i.e. it is uniaxial) birefringence can be formalised by assigning two different
refractive indices to the material for different polarizations. The birefringence magnitude is then defined by where no and ne are the refractive indices for polarizations perpendicular (ordinary) and parallel (extraordinary) to the axis of anisotropy respectively.
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fiz. double-refracting
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