The capacity of a substance to rotate the plane of polarization of plane-polarized light when examined in an instrument known as a polarimeter. All compounds that are capable of existing in two forms that are nonsuperimposable mirror images of each other exhibit optical activity. Such compounds are called stereoisomers (or enantiomers or chiral molecules) and the two forms arise because compounds having asymmetric carbon atoms to which other atoms are connected may arrange themselves in two different ways.
The property possessed by some substances of rotating the plane of polarization of polarized light. A compound that rotates the plane to the left is described as levorotatory (or 1-); one that rotates the plane to the right is described as dextrorotatory (or d-).
In chemistry, the rotation of the plane of polarized light when the light passes through a chemical solution. Measurement of this property, called polarimetry, is useful in the determination of optically active substances such as dextrose. Sugars are classified according to this criterion. Optical activity of a substance in solution can be detected by placing it between polarizing and analyzing prisms.