Optical Activity

A material which rotates the plane of incident linearly polarized light is said to be optically active. Viewing the light head-on, some substances rotate the electric field clockwise (dextrorotatory) and some produce a counterclockwise rotation (levorotatory). The property was discovered in quartz in 1811 by Arago. Two different crystalline structures of quartz produce d-rotatory and l-rotatory behavior. The two crystalline forms are said to be enantiomorphs of each other. The optical activity of quartz is associated with its crystal structure, as evidenced by the fact that neither molten quartz or fused quartz demonstrate optical activity.

In the case of many naturally occurring organic compounds such as sugar, tartaric acid and turpentine, optical activity is exhibited in the liquid state. This shows that the activity is associated with the individual molecules themselves.