A sample of material able to rotate the plane of @P04712@ of a beam of transmitted plane-polarized light is said to possess optical activity (or to be optically active). This @O04312@ is the classical distinguishing characteristic (sufficient but not necessary) of systems containing unequal amounts of corresponding @E02069-1@. An @E02069-2@ 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 @C01064@ phenomena.