Optical isomers are non-superimposable mirror images of each other.Optical isomerism occurs when there are four different groups attached to an asymmetric or chiral carbon atom.In that case the shape would be tetrahedral and there will be two molecules or enantiomers that are non-superimposable images of each other.If a molecule exhibits optical isomerism, synthesis of the molecule from achiral starting materials usually produces both enantiomers in equal quantities. This is called a racemic mixture.