Rotary polarization is an optical phenomenon occurring in certain crystalline materials. The vibrational direction of light passing through such a crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

 (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

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 on the spiral structure of linked silicon-oxygen tetrahedra. Sucrose

Sucrose

Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...

 and camphor

Camphor

Camphor is a waxy, white or transparent solid with a strong, aromatic odor. It is a terpenoid with the chemical formula C10H16O. It is found in wood of the camphor laurel , a large evergreen tree found in Asia and also of Dryobalanops aromatica, a giant of the Bornean forests...

 are d-rotary whereas cholesterol

Cholesterol

Cholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...

 is l-rotary.

Rotary polarization occurs in molecules and crystals that have no crystallographic

Crystallography

Crystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...

 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

François Arago

François Jean Dominique Arago , known simply as François Arago , was a French mathematician, physicist, astronomer and politician.-Early life and work:...

is credited with the discovery of rotary polarization of quartz in 1811.