For most ray directions, both polarizations would be classified as extraordinary rays but with different effective refractive indices.

These rays with differing refractive indices are known as the Ordinary and extraordinary rays.

For rays travelling along an optic axis the speeds of the ordinary and extraordinary rays are equal.

One source, however, gives lavendulan as nearly uniaxial (-), and quotes only two refractive indices, for the ordinary and extraordinary rays.

The refractive index for Canada Balsam is in between the refractive index for the ordinary and extraordinary rays.

Values of 'k' on the sphere correspond to the 'ordinary rays' while values on the ellipsoid correspond to the 'extraordinary rays'.

When an arbitrary beam of light strikes the surface of a birefringent material, the polarizations corresponding to the ordinary and extraordinary rays generally take somewhat different paths.

If the material is birefringent, the refracted ray may split into ordinary and extraordinary rays, which experience different indexes of refraction when passing through the birefringent material.

Light polarized parallel to the axis are called "extraordinary rays" or "extraordinary photons", while light polarized perpendicular to the axis are called "ordinary rays" or "ordinary photons".