The classical electromagnetic theory of light put forward by Maxwell advocates that the electric and magnetic fields associated with a beam of monochromatic light vibrate in all directions perpendicular to the direction of propagation of light. In fact, there exists an indefinite number of planes that pass through the line of propagation, and an ordinary light usually vibrates in all the planes. This is also referred to as unpolarized light. Under certain specific circumstances, the vibrations may all be restricted to one direction only, in the perpendicular plane and this is termed as plane-polarized light.
Figures 19.1 (a) and (b) depict the ordinary or unpolarized light and plane-polarized light respectively.
A few crystalline substances, for instance : Iceland spar, Calcite (a form of CaCO3) or Polaroid, possess different refractive indices for light whose field oscillates either perpendicular or parallel to the principal plane of the crystal. Thus, an ordinary light (unpolarized light) gets converted into a plane-polar-ized light by simply passing it through a lens made of the above cited materials and traditionally called a Nicol prism (after William Nicol-the inventor).
Therefore, an optically active substance is one that rotates the plane of polarized light. In other words, certain specific substances by virtue of their internal structure may be able to transmit only such vibrations that are oriented along certain directions and entirely block vibrations in other directions.
Figure 19.2 evidently shows the electric field of a plane-polarized light which consists of two components of fixed magnitude rotating in opposite directions to one another ; the right circularly polarized light ; and the left circularly polarized light. However, it is worth mentioning that the plane-polarized beam is the vector-sum of these two components.