Explain optical isomerism.

(N/A) Optical isomers are mirror images that cannot be superimposed on each other. These are called enantiomers.
Molecules or ions that cannot be superimposed are called chiral.
The two forms are called dextrorotatory $(d)$ (rotating plane-polarized light to the right) and laevorotatory $(l)$ (rotating plane-polarized light to the left) based on the direction in which they rotate plane-polarized light in a polarimeter.
Optical isomerism is common in octahedral complexes involving bidentate ligands. In a complex of the type $[PtCl_{2}(en)_{2}]^{2+}$,only the cis-isomer shows optical activity.