Polarization by Selective Absorption

Waves emitted by a radio transmitter are usually linearly polarized. The vertical antennas that are used for radio broadcasting emit waves that, in a horizontal plane around the antenna, are polarized in the vertical direction (parallel to the antenna).

The situation is different for visible light. Light from incandescent light bulbs and fluorescent light fixtures is not polarized. The “antennas” that radiate light waves are the molecules that make up the sources. The waves emitted by any one molecule may be linearly polarized, like those from a radio antenna. But any actual light source contains a tremendous number of molecules with random orientations, so the emitted light is a random mixture of waves linearly polarized in all possible transverse directions. Such light is called unpolarized light or natural light. To create polarized light from unpolarized natural light requires a filter.

Figure 3.5. A Polaroid filter is illuminated by unpolarized natural light. The transmitted light is linearly polarized along the polarizing axis.

In 1938, E. H. Land (1909–1991) discovered a material, which he called polaroid, that polarizes light through selective absorption by oriented molecules. This material have dichroism, a selective absorption in which one of the polarized components is absorbed much more strongly than the other (Fig. 3.5) is fabricated in thin sheets of long-chain hydrocarbons. The sheets are stretched during manufacture so that the long-chain molecules align. After a sheet is dipped into a solution containing iodine, the molecules become good electrical conductors. However, conduction takes place primarily along the hydrocarbon chains because electrons can move easily only along the chains.

As a result, the molecules readily absorb light whose electric field vector is parallel to their length and allow light through whose electric field vector is perpendicular to their length. It is common to refer to the direction perpendicular to the molecular chains as the transmission axis. Polaroid widely used for sunglasses and polarizing filters for camera lenses. A Polaroid filter transmits 80% or more of the intensity of a wave that is polarized parallel to a certain axis in the material, called the polarizing axis, but only 1% or less for waves that are polarized perpendicular to this axis.

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