Light consists essentially of electromagnetic waves having a sinusoidally changing electric field at right angles to, and in phase with, a sinusoidally changing magnetic field. An electromagnetic wave is diagrammatically represented in figure 2. The physical chemical effects of light (e.g. photosynthesis, light absorption, photo emission, polarization etc.) are generally attributed to the electric vector of the electromagnetic wave. The term polarization is given to the angle the electric vector makes with the normal and, in the figure 2, it is seen that the light is depicted as vertically

polarized. That is to say all the electric vector vibrations occur in the vertical plane. Light emitted from a source is normally not polarized at all, and the axis of the electric vector can take all angles to the normal.

Figure 2. An Electromagnetic Wave

Optical activity is a unique characteristic of specific enantiomers and, thus, its measurement is important and is carried out by means of a Polarimeter (shown in figure 3) that will be briefly considered.

Unpolarized light from a suitable source is made to pass through a polarizer that usually consists of a pair of crossed Nichol prisms. The light leaving the polarizer is plane polarized and, in the diagram, the angle of the Nichol prisms has been adjusted to produce vertically polarized light. This light is then passed through a tube containing the sample. If the sample is optically active, the plane of the polarized light will be rotated, as shown in figure 3. The extent of rotation will depend on the polarizability of the substance and its concentration. The light from the sample cell is then passed through another pair of crossed Nichol prisms called the analyzer. The analyzer prisms are then rotated so that the transmitted light is again vertically polarized. The angle through which the analyzer has been turned is called the angle of polarization. Clockwise rotation of polarized light is designated as (+) and anti clockwise rotation as ( ). The established method of defining rotation is that the (+) isomer is termed dextro and the ( ) isomer laevo.