Birefringence
![Incoming light in the parallel (s) polarization sees a different effective index of refraction than light in the perpendicular (p) polarization, and is thus refracted at a different angle.](/Images/godic/202501/10/Positively_birefringent_material.svg5213.png")
![A calcite crystal laid upon a graph paper with blue lines showing the double refraction](/Images/godic/202501/10/Crystal_on_graph_paper5213.jpg")
![Doubly refracted image as seen through a calcite crystal, seen through a rotating polarizing filter illustrating the opposite polarization states of the two images.](/Images/godic/202501/10/Calcite_and_polarizing_filter5213.gif")
![Reflective twisted nematic liquid crystal display. Light reflected by surface (6) (or coming from a backlight) is horizontally polarized (5) and passes through the liquid crystal modulator (3) sandwiched in between transparent layers (2,4) containing electrodes. Horizontally polarized light is blocked by the vertically oriented polarizer (1) except where its polarization has been rotated by the liquid crystal (3), appearing bright to the viewer](/Images/godic/202501/10/LCD_layers.svg5213.png")
Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with asymmetric crystal structures are often birefringent, as are plastics under mechanical stress.