Fr.: activité optique
The property possessed by some substances and their solutions of rotating the plane of vibration of → polarized light. When a beam of → linearly polarized light is sent through an optically active substance, such as crystalline quartz and sugar solution, the direction of vibration of the emerging linearly polarized light is found to be different from the original direction. Those which rotate the → plane of polarization to the right, for an observer looking in the incoming beam, are called → dextrorotatory or right handed; those which rotate it to the left, → levorotatory or left handed. Optical activity may be due to an asymmetry of molecules of a substance (solutions of cane sugar) or it may be a property of a crystal as a whole (crystalline quartz).