Rotary polarization is an optical phenomenon occurring in certain crystalline materials. The vibrational direction of light passing through such a crystal (or sometimes liquids) is rotated so that it varies in a circular manner. The amount of polarization rotation depends on the thickness of the crystal. The rotation may be either to the right (dextrorotary - d-rotary) or left (levorotary - l-rotary) depending, for example, in quartz on the spiral structure of linked silicon-oxygen tetrahedra. Sucrose and camphor are d-rotary whereas cholesterol is l-rotary.
Rotary polarization occurs in molecules and crystals that have no crystallographic mirror planes or inversion centers. Such crystals are enantiomorphous (the existence of two chemically identical crystal forms as mirror images of each other). Crystalline quartz SiO2 exhibits rotary polarization whereas the amorphous forms of silica do not. Quartz plates are used in optical crystallographic microscopy.
François Arago is credited with the discovery of rotary polarization of quartz in 1811.