A sample of material able to rotate the plane of @[email protected] of a beam of transmitted plane-polarized light is said to possess optical activity (or to be optically active). This @[email protected] is the classical distinguishing characteristic (sufficient but not necessary) of systems containing unequal amounts of corresponding @[email protected] An @[email protected] causing rotation in a clockwise direction (when viewed in the direction facing the oncoming light beam) under specified conditions is called dextrorotatory and its chemical name or formula is designated by the prefix (+)-; one causing rotation in the opposite sense is laevorotatory and designated by the prefix (−)-. Materials with optical activity also exhibit other @[email protected] phenomena.