optical activity. Some crystals and liquids rotate the plane of polarization of plane polarized light as it travels through the medium. This phenomenon is known as optical activity. The substances that produce such effects are called optically active. Quartz crystal and sugar solution are common examples. A clockwise rotating substance is called dextrorotatory and an anti clockwise rotating substance is called levorotatory.

The angle of rotation a for a solution is proportional to the concentration, C and the distance through which light ray travels in the liquid,

a = s C l (o2)

where the constant s is called the specific rotation. Specific rotation is defined as rotation produced for unit concentration of the liquid (1 g/ml of solvent), and 10 cm of path length.