Optical activity

When an aqueous solution of certain organic compounds -- such as sugar or lactic acid -- is placed in the path of plane polarised light, the light that emerges on the other side has its oscillations in a different plane than the light that went in: the incident light's plane of polarisation has been rotated, either to the right (clockwise, as seen by an observer facing the oncoming light) or to the left (anticlockwise). This property, by which a substance rotates the plane of polarisation of plane polarised light passed through it, is called optical activity. A compound that shows this rotation is called optically active; one that does not is optically inactive. The rotation is expressed numerically as the optical rotation -- the actual angle through which the substance turns the plane of polarisation -- and, depending on the direction, an optically active compound is described as dextrorotatory (turns the plane to the right; symbol d- or (+)) or laevorotatory (turns the plane to the left; symbol l- or (-)). Isomerism in which isomeric compounds differ in their optical activity is called optical isomerism. The French scientist Louis Pasteur was the first to recognise that optical activity is associated with a particular kind of three-dimensional molecular structure, and he introduced the term 'enantiomers' for optical isomers that show equal but opposite optical rotation. It is worth stressing the logical order here: optical activity is an experimen …