Optical isomerism is a form of isomerism (specificly geometric isomerism) whereby the different 2 isomers are the same in every way except being non-superimposable mirror images(*) of each other. Optical isomers are known as chiral molecules (prounounced ki-rall) .
First discovered by Louis Pasteur nineteenth century, the study of optical isomerism is called stereochemistry. Optical isomers are often called stereoisomers (in fact, stereoisomers constitute a more general group, since stereoisomerism needn't necessarily imply optical activity).
Two types of molecules which differ only in their relative stereochemistry are said to be enantiomers of each other. A mixture of equal amounts of both enantiomers is said to be a racemic mixture.
This form of isomerism can arise when an atom (usually carbon) is surrounded by four different functional groups. Swapping two of the groups round can arise in two different molecules - mirror images of each other.
A rule for determining which isomeric form (usually called D-... and L-...) an amino acid is, is called the CORN rule. The groups:
COOH, R, NH2 and H (where R is an unnamed carbon chain) -- - -
are arranged round the chiral center C-atom. If the above groups are arranged clockwise round the atom then it is the L-form. Anticlockwise means the D-form.
Properties of optical isomers
They are identical with respect to ordinary chemical reactions, but differences arise when they are in presence of other chiral molecules. For example Spearmint chewing-gum and Caraway seeds respectively contain L-carvone and D-carvone - enantiomers of carvone. These smell differently to most people because our taste receptors also contain chiral molecules which behave differently in the presence of different enantionmers.
D-form Amino acids tend to taste sweet whereas L-forms are usually tasteless. This is again due to our chiral taste molecules. The smells of oranges and lemons are examples of the D and L enantiomers.
Penicillin works by stereoisomeric means. The antibiotic only works on peptide links of D-alanine which occur in the cell walls of bacteria - but not in humans. The antibiotic can only kill the bacteria and not us because we don't have these D-amino acids.
The anti-nausea drug Thalidomide was widely prescribed to pregnant women until it was linked to birth defects. It was later discovered that one enantiomer was responsible for the teratogenic effects.
(*)The term non-superimposable distinguishes mirror images which are superimposable, such as the mirror image of the letter "A", on the original, from those that aren't. The classic example of this are human hands--the left hand is a non-superimposable mirror image of the right hand: No matter how the two hands are oriented relative to one another, one cannot line up all the major features of one hand with the other, whereas such an operation is trivial for a non-chiral mirror image (eg, our letter "A").