Birefringence
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Birefringence is the numerical difference between the refractive indices in an anisotropic crystal. It appears as a band of polarization colors under crossed polars.

birefringence = n2 - n1

where n2 is the higher refractive index
and n1 is the lower refractive index.

(See isotropic versus anisotropic, also refractive index)