Optical isomerism

 Optical isomerism is a form of Stereoisomerism.  It arises due to the presence of a Chiral_carbon, a carbon atom attached to four different groups.  This causes two non-superimposable mirror images to be formed, these are known as enantiomers.  They can be distinguised using plane polarised light, with the (+) form rotating the light in a clockwise direction and the (-) form rotating the light in an anti-clockwise direction.