optical activity The ability of certain substances to rotate the plane of plane-polarized light as it passes through a crystal, liquid, or solution. It occurs when the molecules of the substance are asymmetric, so that they can exist in two different structural forms each being a mirror image of the other. The two forms are optical isomers or enantiomers (see illustration). The existence of such forms is also known as enantiomorphism (the mirror images being enantiomorphs). One form will rotate the light in one direction and the other will rotate it by an equal amount in the other. The two possible forms are described as dextrorotatory or laevorotatory according to the direction of rotation, and the prefixes (+)- and (–)- are used, respectively, to designate the isomer, as in (+)-tartaric and (–)-tartaric acids. (The prefixes d- and l- are now obsolete.) An equimolar mixture of the two forms is not optically active. It is called a racemic mixture (or racemate) and designated by (±)-. In addition, certain molecules can have a meso form in which one part of the molecule is a mirror image of the other. Such molecules are not optically active.

Molecules that show optical activity have no plane of symmetry. The commonest case of this is in organic compounds in which a carbon atom is linked to four different groups. An atom of this type is said to be a chiral centre. Many naturally occurring compounds show optical isomerism and usually only one isomer occurs naturally. For instance, glucose is found in the dextrorotatory form. The other isomer, (–)-glucose, can be synthesized in the laboratory, but cannot be synthesized by living organisms.

optical activity The ability of some compounds to rotate the plane of polarized light because of the asymmetry of the molecule. If the plane of light is rotated to the right, the substance is dextrorotatory and is designated by the prefix (+); if laevorotatory (rotated to the left), the prefix is (−). A mixture of the two forms is optically inactive and is termed racemic.

Sucrose is dextrorotatory but is hydrolysed to glucose (dextrorotatory) and fructose, which is more strongly laevorotatory so hydrolysis changes optical activity from (+) to (−); hence, the mixture of glucose and fructose is termed invert sugar.

(The obsolete notation for (+) was d‐ and for (−) was l‐; this is distinct from D‐ and L‐, which are used to designate stereo‐isomerism; see D‐.)