optical activity, the ability of asymmetric compounds to rotate the orientation of planar polarized light. Such compounds and their mirror images are know as enantiomers, or optical isomers. Although differing in geometric arrangement, enantiomers possess identical chemical and physical properties. Since each type of enantiomer affects polarized light differently, optical activity can be used to identify which enantiomer is present in a sample and its purity. Certain molecular groups, known as chromophores, possess high optical activity due to mobile electrons that interact with light and are responsible for the color of certain objects (e.g. chlorophyll chromophore). Optical activity is measured by two methods: optical rotation, which observes a sample's effect on the velocities of right and left circularly polarized light beams; and circular dichroism, which observes a sample's absorption of right and left polarized light. See also polarization of light.

Ability of a substance to rotate the plane of polarization of a beam of light passed through it, either as crystals or in solution. Clockwise rotation as one faces the light source is “positive,” or dextrorotary; counterclockwise rotation “negative,” or levorotary. Louis Pasteur was the first to recognize that molecules with optical activity are stereoisomers (see isomerism). Optical isomers occur in pairs that are nonsuperimposable mirror images of one another. They have the same physical properties except for their effect on polarized light; in chemical properties they differ only in their interactions with other stereoisomers (see asymmetric synthesis).

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