Vibrational direction of light passing through a crystal (or sometimes liquids) is rotated so that it propagates in a helical manner either to the right (dextrorotary) or left (levorotary).
Occurs in molecules and crystals that have no mirror planes or inversions centers. Such crystals are enantiomorphous and must exist in one of eleven enantiomorphic point groups. E.g., Quartz SiO2, crystallizes in either P312(d) or P322(l). Tartaric acid "molecules" have only a 2-fold symmetry axis and are also either d or l.