The optical isomers which do not mirror images of each other are called diastereoisomers and this phenomenon is called diastereoisomerism.
Their characteristics are given below:
1. These isomers can not superimposable to each other.
2. Diastereoisomerism is found only, in those compounds in which at least two chiral carbon centres are present.
3. The physical properties like melting point, boiling point, solubility, spectre rotation etc of diastereoisomers of a compound are different. So, these can be separated by fractional distillation, fractional crystallisation and chromatographic methods easily.
Example:
Cinnamic acid dibromide
Here, I & II and III & IV one enantiomers as there are mirror images of each other. However, I & II, I & IV, II & III and II & IV are diastereomers.