Isomers are molecules that have the same molecular formula but the bonds are arranged differently in space.
Optical isomerism refers to two isomers that are mirror images of each other and non-superimposable (they cannot be placed on top of each other to make a perfect fit, in the same way as placing one hand on the other does not produce tessellation). Molecules that fit this description are referred to as enantiomers. When enantiomers are placed in symmetrical environments, they have the same chemical properties and physical properties (such as melting and boiling points) but they rotate plane-polarised light in equal amounts though in opposite directions.
Enantiomers that rotate plane-polarised light in a clockwise direction are know as L-(+)-enantiomers or (S)-enantiomers while those which rotate plane-polarised light in an anti-clockwise direction are known as D-(-)-enantiomers or (R)-enantiomers.
The L in these terms means laevorotatory, and the D means dextrorotatory.
A sign of optical isomerism is a chiral centre. This refers to a carbon atom that is bonded to 4 different groups, this causes a lack of symmetry and consequently is known as an asymmetric carbon atom.
In the example above, notice the 4 different groups attached to the central carbon atom
Mixtures containing 50% of each enantiomer are called racemic mixtures and overall, have no effect on plane-polarised light as each enantiomer cancels the other out.
An example of enantiomers are the antidepressant drugs escitalopram and citalopram. Citalopram is a racemic mixture (or racemate) containing (S)-citalopram and (R)-citalopram, whereas escitalopram contains only (S)-citalopram. As the (S)-enantiomer produces the desired antidepressant effects, escitalopram is prescribed in 1/2 the dosage that citalopram was.