optical activity

In chemistry, the ability of certain crystals, liquids, and solutions to rotate the plane of polarized light as it passes through them. The phenomenon is related to the three-dimensional arrangement of the atoms making up the molecules concerned. Only structures that lack a plane of symmetry exhibit optical activity.

Such asymmetric molecules are called chiral (from the ancient Greek for ‘hand’. The two versions of a chiral molecule that are mirror-images of each other are known as enantiomers and will display optical activity of equal magnitude but opposite direction. A mixture of both versions, where the optical activity is canceled out, is called a racemate.

The connection between optical activity and molecular structure was first recognized by Louis Pasteur. In 1848, he crystallized the optically inactive ‘racemic acid’ (hence the term ‘racemate’). By sorting the crystals according to their asymmetric shapes, he obtained the optically active enantiomers of tartaric acid.