Linked from
The 28 pages that link to Birefringence, each with the reason it gives.
CalciteRelated: Calcite’s strong birefringence produces a conspicuous double image.
PolarizationRelated: It causes different polarization components to travel at different speeds.
Jones calculusRelated: It creates phase delays between components, modeled by Jones matrices.
Liquid crystalRelated: Aligned molecules create direction-dependent refractive indices.
SapphireRelated: Corundum’s optical anisotropy affects how light travels through sapphire.
Crystal opticsRelated: It is the defining propagation effect in many anisotropic crystals.