Optical activity is the phenomenon that the polarization direction of light is gradually rotated clockwise (dextrorotary) or anti-clockwise (levorotary) when light passes through certain transparent substances. That can occur when a substance contains chiral molecules and has different concentrations of the two enantiomers, i.e., the two versions of those molecules which are mirror images of each other, and rotated the polarization in opposite directions. An example for that situation is ordinary sugar (sucrose), which contains only one of the two possible enantiomers, the dextrorotary version. That striking asymmetry is related to the fact that biological organisms producing sugar have the same kind of asymmetry for a number of other types of molecules, and that the relative chirality of molecules can substantially influence their chemical reaction dynamics.
One can describe optical activity as a difference of the velocity of light (phase velocity) between two opposite rotation directions of circularly polarized light. Similarly, the phenomenon of different Raman scattering intensities for left and right polarized light is called Raman optical activity.
While optical activity is usually related to the presence of chiral molecules, as explained above, it can also be induced by a magnetic field in some substances; that is called the Faraday effect, which is exploited particularly in Faraday rotators and Faraday isolators.
Measurement of Optical Activity
The magnitude of optical activity can be measured with a polarimeter. While the basic measurement principle is rather old, these instruments have been substantially revised over the years and now allow for highly accurate measurements. There are even automated polarimeters which are for the combination of convenience, high speed and high accuracy.