Xenotime has two directions of perfect prismatic dichroic with pink, yellow or yellowish brown seen in the extraordinary ray and brownish yellow, grayish brown or greenish brown seen in the ordinary ray.
12.
Therefore, for any arbitrary direction of propagation ( other than in the direction of the optic axis ), two distinct wavevectors "'k "'are allowed corresponding to the polarizations of the ordinary and extraordinary rays.
13.
So even in the case of normal incidence, where the angle of refraction is zero ( according to Snell's law, regardless of effective index of refraction ), the energy of the extraordinary ray may be propagated at an angle.
14.
This solution corresponds to the " extraordinary rays " in which the effective refractive index is in between " n o " and " n e ", depending on the direction of "'k " '.
15.
Light traveling in the forward direction is split by the input birefringent wedge into its vertical ( 0?) and horizontal ( 90?) components, called the ordinary ray ( o-ray ) and the extraordinary ray ( e-ray ) respectively.
16.
In addition, a distinct form of double refraction occurs in cases where the optic axis is not along the refracting surface ( nor exactly normal to it ); in this case the electric polarization of the birefringent material is not exactly in the direction of the wave's electric field for the extraordinary ray.
17.
The difference between the refractive indexes of the ordinary and the extraordinary ray in quartz is . 009, and in a rock-section about 1 / 500 of an inch thick this mineral gives grey and white polarization colours; nepheline with weaker double refraction gives dark grey; augite on the other hand will give red and blue, while calcite with the stronger double refraction will appear pinkish or greenish white.
18.
The different angles of refraction for the two polarization components are shown in the figure at the top of the page, with the optic axis along the surface ( and perpendicular to the plane of incidence ), so that the angle of refraction is different for the " p " polarization ( the " ordinary ray " in this case, having its polarization perpendicular to the optic axis ) and the " s " polarization ( the " extraordinary ray " with a polarization component along the optic axis ).
