Optical isomers are stereoisomers where the molecules are mirror images of each other. The two molecules below are optical isomers: even though they look the same at first glance, you won't be able to rotate them so that they look the same. Molecules that exhibit optical isomerism are known as chiral. The two mirror images are known as enantiomers.
In general, molecules that have four different groups joined to the same atom exhibit optical isomerism. The molecule below has a hydrogen, fluorine, chlorine and bromine atom joined to the central carbon. Chiral molecules rotate polarized light.

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