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
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.