Editing Birefringence (section)

=== Fast and slow rays ===
In a birefringent material, a wave consists of two polarization components which generally are governed by different effective refractive indices. The so-called ''slow ray'' is the component for which the material has the higher effective refractive index (slower phase velocity), while the ''fast ray'' is the one with a lower effective refractive index. When a beam is incident on such a material from air (or any material with a lower refractive index), the slow ray is thus refracted more towards the normal than the fast ray. In the example figure at top of this page, it can be seen that refracted ray with ''s'' polarization (with its electric vibration along the direction of the optic axis, thus called the extraordinary ray<ref>Born & Wolf, 2002, pp.{{nnbsp}}807–808. (In 19th-century terminology, the ordinary ray is said to be polarized in the plane of the optic axis; but this "[[plane of polarization]]" is the plane ''perpendicular'' to the vibration; cf.&nbsp;Fresnel, 1827, tr.&nbsp;Hobson, p.{{nnbsp}}318.)</ref>) is the slow ray in given scenario.

Using a thin slab of that material at normal incidence, one would implement a [[waveplate]]. In this case, there is essentially no spatial separation between the polarizations, the phase of the wave in the parallel polarization (the slow ray) will be retarded with respect to the perpendicular polarization. These directions are thus known as the slow axis and fast axis of the waveplate.