Optical isomers

Optical isomers rotate the plane of plane polarised light by 90° clockwise or anticlockwise. Optical isomers will have a central, chiral carbon atom. Chirality depends on the central C atom having four different groups attached to it as this makes the molecule superimposabe on its mirror image (asymmetrical). Optical isomers (enantiomers) rotate the plane of plane polarised light in opposite directions. 

Two enantiomers of an optical isomer will have very similar physical and chemical properties making it very hard to tell the difference between enantiomers.