Ordinary and extraordinary rays in a uniaxial crystal (calcite)
In a uniaxial crystal like calcite, the ordinary ray (o-ray) obeys Snell's law with index n_o regardless of direction. The extraordinary ray (e-ray) has an index that varies from n_o (along the optic axis) to n_e (perpendicular to it). The walk-off angle δ = arctan(Δn·tan(θ)/n²) separates the two beams. Calcite (n_o=1.658, n_e=1.486) gives Δn=0.172 — one of the largest natural birefringences.