(Science: chemistry) optical property of a material in which the refractive index is different for light polarized in one plane compared to the orthogonal plane.
May arise from molecular organisation of the material (form birefringence.), alignment of molecules due to tension (stress birefringence.) or alignment of rod like particles in flow (flow birefringence).
With crossed Nicoll prisms a birefringent material appears bright against a dark background.