Birefringence
Birefringence (or double refraction) happens when light is passed through a special material and it splits into two different rays of light. Calcite crystals and boron nitride are two good examples of materials that cause birefringence. Just as a prism splits light into a large number of colors, birefringence splits light into two images of the same color. You see this as a "double image".
Birefringence Media
A calcite crystal laid upon a graph paper with blue lines showing the double refraction
Doubly refracted image as seen through a calcite crystal, seen through a rotating polarizing filter illustrating the opposite polarization states of the two images.
Comparison of positive and negative birefringence : In positive birefringence (figure 1), the ordinary ray (p-polarisation in this case w.r.t. magenta-coloured plane of incidence), perpendicular to optic axis A is the fast ray (F) while the extraordinary ray (s-polarisation in this case and parallel to optic axis A) is the slow ray (S). In negative birefringence (figure 2), it is the reverse.
Sandwiched in between crossed polarizers, clear polystyrene cutlery exhibits wavelength-dependent birefringence
Color pattern of a plastic box with "frozen in" mechanical stress placed between two crossed polarizers
Birefringent rutile observed in different polarizations using a rotating polarizer (or analyzer)
Surface of the allowed k vectors for a fixed frequency for a biaxial crystal (see eq. 7).
Other websites
Media related to Birefringence at Wikimedia Commons
