Polarization

What happens when a beam of ordinary light passes through certain crystals? Atoms in a crystal are arranged in a large number of parallel slots. Light will pass through both crystals when their slots are parallel, but will be completely cut off if the slots are crossed.

Fig 108 Credits: Freeman

A single crystal will then hold back all the vibrations except the one that is lined up with its own grain. A beam of light whose vibrations are thus confined to one direction is said to be plane-polarized. This setup also shows that light waves are transverse. Longitudinal waves cannot be polarized.

FIG 108

An invention by Nicol can be used to produce and detect polarized light. This is known as Nicol prism. The prism is held in front of the beam of light and is rotated. If the beam is plane polarized the light seen through the Nicol Prism varies in intensity and none passes through at one position of the prism.

Polaroids are used in many practical applications of polarized light. For example they are used in sunglasses to reduce the intensity of incident sunlight and to eliminate reflected light or glare from the road.

Polarized light can be used to find out just how the stresses are distributed in machine parts. A model of the part is made out of plastic and subjected to the kind of stress the original would get in actual use. When viewed by polarized light., color bands appear which show the exact stress pattern in the piece.

Remember the Nicol Prism? It is used in a saccharimeter. Saccharimetry is the measurement of concentrations of sugar. Due to the molecular structure of the sugar, these solutions rotate the plane of polarization of plane-polarized light as the light passes through. The rotation of the plane of the polarization when the incident light is viewed may be right-handed (clockwise) or left-handed (anti-clockwise).

Polarimetry is the science concerned with the angle of rotation of plane-polarized light. It is important in chemistry since many chemical compounds are optically active; they have the power of rotating the plane of polarization of a beam of polarized light. The phenomenon occurs when the molecular structure of the compound lacks symmetry so that the molecule and its mirror image are not superimposable. Polarimetry has important applications in the sugar industry, since sucrose is far more optically active than many of the more common impurities and so polarimetry can be used to measure the purity of sugar.

Polarimetry is also used to characterize and distinguish stereoisomers, which are compounds with the same composition and structure, but different configurations of atoms within the molecule. Two well known streoisomers are tartaric and racemic acids.Tartaric acid rotates the plane of polarized light to the right while racemic acid is optically inactive, suggesting different symmetries within the two molecules. While these acids differ in many other observable properties, it is often the case that stereoisomers can only be distinguished by polarimetry and by their reactions with other optically active substances.