Birefringent materials (like certain crystals and minerals) polarize and split the light into two separate rays that take different paths through the crystal and travel with different speeds. When viewed between crossed polarizers (dark background) they show interference colors that vary with the thickness of the crystal and its orientation. The image shows a sodium citrate crystal between two linear polarizers that are perpendicular to each other (crossed) to provide a black background. Field width: 0.6 mm. Image by Eva Johannes, Cellular and Molecular Imaging Facility.