English: Titanium dioxide, or rutile, forms birefringent crystals. When polarized light passes through, its polarisation changes to elliptical and with a different main axis than was the original polarisation axis. This is approximately 1 mm long, 0.5 mm thick piece of monocrystalline TiO2, photographed with crossed polarizer and analyzer. For the polarizer angles of 45, 135, 225 or 315 degrees, the polarisation change is largest, passing the most light through the crystal. Note that this phenomenon is wavelength dependent, creating colour spots on the edges.