Polarized Light

Polarized light consists of light waves that have a simple, orderly arrangement. The waves of ordinary light are arranged in a complex, disorderly manner. Ordinary light from the sun or a lamp is composed of disorderly waves that vibrate in all directions perpendicular to the light beam. But polarized light consists of orderly waves that vibrate in only one direction. Because of its orderly structure, polarized light can be used in ways in which ordinary light cannot. For example, the internal physical structure of many transparent materials can be seen with the aid of polarized light. Light polarizers are powerful tools that are used in science, industry, and daily life.

How light is polarized. To understand polarization, think of a light beam as a train of electromagnetic waves. The electromagnetic forces making up these waves vibrate in a direction perpendicular to the path of the beam. A rough example of these waves can be made by attaching a rope to a wall and shaking the other end. A train of waves will move along the length of the rope. Each part of the rope will vibrate in all directions perpendicular to its length. Waves that vibrate in this way are called transverse waves. See Waves.

Polarized light vibrates in a single direction perpendicular to its path. Ordinary light can be polarized by passing it through a special light polarizing filter. This filter allows only the waves that vibrate in one perpendicular direction to pass through. The structure of the light polarizing filter prevents the passage of light waves that vibrate in other perpendicular directions. In scientific terms, the light polarizing filter allows the components (parts) of the light waves that vibrate in one vibration-direction to pass through. The components of waves that vibrate in all other directions are held back. The light that passes through the light polarizing filter is called polarized light.

All the vibrations that pass through a polarizing filter vibrate in one direction--a direction parallel to the optical grain of the filter. The optical grain is the transmission axis of the filter. Polarized light can pass completely through a second polarizing filter whose transmission axis is parallel to that of the first polarizing filter. But if the second polarizer is rotated like a wheel, it will gradually dim the light that comes through it. The second polarizer will cut off the light entirely when its axis is "crossed" at 90º to the axis of the first polarizing filter. The dimming and cutting off occur because each polarizing filter absorbs all components of the light that do not vibrate parallel to the filter's axis. As a result, the brightness of the light beam is gradually reduced as the transmission axis of the second polarizer cuts across the transmission axis of the first.

Many applications of polarized light are based on this phenomenon. For example, much of the light around us is already polarized. Mirrorlike reflections from shiny horizontal surfaces, such as pavement and water, consist largely of light that has been horizontally polarized in the process of reflection. Polarized sunglasses, with their transmission axis set vertically, block the horizontally polarized light making up the bright reflections. Photographers use polarizing filters to cut down glare and reflections from shiny surfaces such as windows and water.

Polarizing materials. The most widely used light polarizers consist of thin plastic sheets. A typical plastic sheet contains millions of long, slender, carefully aligned chains of iodine molecules. Each of these chains acts like an individual polarizing filter. Sheet polarizers have greatly extended the uses of polarized light because of their low cost and convenient size. Edwin H. Land, inventor of the Polaroid Land camera, invented the first sheet polarizer in 1928, when he was 19 years old.

Some natural crystals, such as tourmaline, can polarize light. Tourmaline transmits the components that lie in one vibration-direction, and holds back others by absorbing them internally. Another natural polarizing crystal is calcite, or Iceland spar. It divides the light into two polarized beams that are at right angles to each other. Nicol prisms are cut from Iceland spar so that one of these beams is eliminated.

Uses of polarized light. Scientists have suggested that polarized glass be used for car headlights and windshields to prevent driving glare from the lights of approaching cars.

Scientists can study the structure of many transparent materials with the aid of crossed polarizing filters. Microscopes equipped with polarizers show many colorless crystals and biology specimens in brilliant color. A polariscope, an instrument equipped with polarizers, is used to find strains (weak spots) in glass objects such as eyeglasses and laboratory glassware. Chemists can tell the type and amount of sugar in a solution by using a saccharimeter, a type of polariscope. Special polarizing filters that produce circularly polarized light are used on radarscopes to trap unwanted reflections.

Contributor: Brian J. Thompson, Ph.D., Provost Emeritus, Distinguished Univ. Prof., and Prof. of Optics, Univ. of Rochester.

Polarized Light

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