optical activity - (n.)
the property of some crystals, gases, liquids, and solutions to rotate plane-polarized light to the left or right. It occurs because the molecules that make up the substance through which the light beam is shone are asymmetric, i.e., they have no plane of symmetry. Asymmetric molecules are mirror images of each other that cannot be superimposed. This asymmetric property is also referred to as handedness. Examples are the L- and D-forms of amino acids.