What is meant by the term optical isomerism?

Optical isomerism is a form of stereoisomerism (different arrangement of the same atoms in 3D space) which is caused by the presence of a chiral carbon in a molecule. A chiral carbon is a carbon with four different groups attached to it, for example a carbon bonded to a hydrogen atom, carboxylic acid group, amino group and a methyl group is a chiral carbon. The optical isomers of a chiral molecule are known as enantiomers and these enantiomers are non super-imposable mirror images of each other. Enantiomers will rotate plane-polarised light in opposite directions but by an equal amount, so when equal numbers of enantiomers are present in a mixture (known as a racemic mixture), no rotation of plane-polarised light is observed.