The activity is called dextrorotatory if the rotation is clockwise when facing the light, and laevorotatory when it is anticlockwise. It occurs when a molecule exists in two mirror-image forms that cannot be superimposed on one another. Various methods exist for denoting the molecular configuration of optical isomers. Note that the d-form of a compound need not necessarily be dextrorotatory. A mixture containing both isomers in equal proportions is called a racemic mixture .
Optical activity occurs in molecules possessing chirality .
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