Light is an electromagnetic wave, i. e., it is a traveling wave moving due to electric and magnetic field oscillating in two mutually perpendicular planes and its direction of motion is perpendicular to both the electric and magnetic fields. Normal light is a mixture of waves with electric fields oscillating in several different planes perpendicular to the direction of motion. This is called unpolarized light. However, it is possible to use a polarizer like a Nicol prism to convert the unpolarized light beam to a polarized light beam with all electric oscillations lying on the same plane (and similarly magnetic oscillations on a single perpendicular plane).
There are some molecules and crystals which can cause optical rotation when the light travels through these materials. Optical rotation (or optical activity) is the rotation of the orientation of the plane of a polarized light about the direction of light motion.
Most natural compounds are optically active and their activity is related to their stereochemistry (i. e., 3-dimensional arrangement of atoms in a molecule). These materials have asymmetrical structures. For example, some organic molecules (e.g., fructose) have unique spatial distribution of atoms that gives rise to ‘non-superimposable mirror image’ structures. These molecules are known as chiral molecules. When the plane polarized light passes through a chiral medium, interaction of light wave with the asymmetric molecules causes the plane of oscillation to rotate from its original incident orientation. Some molecules tend to rotate the plane of polarisation to the right (dextrorotatory (d) or (+)-compounds) and others rotate the plane of light to the left (levorotatory (l) or (-)-compounds). The magnitude of angular rotation (measured in degrees) depends on the nature and number (or concentration) of the chiral molecules, wavelength of the monochromatic light (the yellow sodium D line near 589 nm wavelength is commonly used), optical path length, and temperature.
Go back
