Light, like radio transmissions and nuclear radiation, is emitted in the form of electromagnetic waves. Specifically, light travels in transverse waves--that is, while traveling along a generally straight path, light waves will vibrate in many directions perpendicular to that path. However, there are several methods available to filter light, causing it to vibrate in only one plane perpendicular to the path of travel. Such filtered light is called polarized light.
When an electron is excited it will vibrate; if excited further, it will sometimes emit a photon of light. Since the excited electron vibrates in only one direction, that photon will also vibrate along a single plane and, therefore, be polarized. If all electrons in a material vibrated in the same direction, all the light emitted would be plane-polarized. In reality, though, the material's electrons vibrate in an almost infinite number of directions, producing unpolarized light. In order for ordinary light to be polarized it must either pass through or bounce off a polarizing substance. Depending upon its individual properties, this substance will either align the light along a single plane or will remove the light not vibrating along that plane.
The first person to observe the polarization of light was Etienne-Louis Malus (1775-1812), who found that light passing through a piece of Iceland spar crystal was split into two beams. Thinking that each beam was aligned with some mystical "pole of light" (similar, in theory, to the poles of a magnet) Malus described the two beams as being "polarized." More precise work on the subject was conducted by a classmate of Malus', Jean-Baptiste Biot (1774-1862), during the final years of the eighteenth century.
Biot's research was further advanced by Augustin Jean-Fresnel (1788-1827), who used the phenomenon to redesign lenses for lighthouses and, more importantly, to support his theory of the transverse nature of light. At this time, debate raging between Europe scientists over whether to subscribe to Isaac Newton's (1642-1727) particle theory of light or, like Fresnel, to the much-doubted theory that light acted as a wave.
In 1828 the Scottish physicist William Nicol (1768-1851) used two pieces of Iceland spar to construct a polarizing prism. As light entered the prism it was split into two beams (just as Malus had observed); upon reaching the second crystal one beam would exit the prism while the second, now polarized, would pass through. This was the first reliable instrument for obtaining polarized light, and, through its use, a generation of scientists began to truly understand the nature of light.
Today, there are four basic methods for polarizing light: absorption, reflection, double refraction, and scattering. The most popular method, absorption, requires a material whose molecules allow only one plane of light to pass through, absorbing all other planes. Such a material is called a dichroic. A common man-made dichroic is polaroid, discovered in 1938 by the American entrepreneur E. H. Land. Land figured that, if a large crystal could be used to polarize light, then thousands of tiny crystals should be capable as well. He mixed the crystals into a clear plastic solution which was cooled and stretched into thin sheets; this had the effect of aligning all the crystals and keeping them from drifting, which is a problem in single-crystal systems. Within a few years polaroid replaced nearly all polarizing crystals in scientific and industrial use, and Land built his Polaroid Corporation from its revenues.
Polarization of light by reflection is found more in nature than in industry. When light strikes a flat surface it is polarized to some extent, depending on the angle at which it strikes the surface. If the angle is zero or 90 degrees, the light will not be polarized; if it is less than 90 degrees, some polarization will occur. At one particular angle light will be completely polarized. This phenomenon is called Brewster's law (after its discoverer, Sir David Brewster [1781-1868]) and the polarizing angle is often referred to as Brewster's angle. Many different materials, such as water, snow, and glass, can polarize light through reflection, though the angle at which this occurs is different for each material.
When Nicol perfected his polarizing prism, he was using the method called double refraction. Certain crystals will split incoming light into two separate rays; Iceland spar is one such crystal, as are calcite and quartz. Each ray produced is polarized, although they are aligned perpendicular to each other--if one ray vibrated up-and-down, the other would vibrate side-to-side. Such polarizing crystals, called birefringent crystals, are seldom used for polarization today, for they are far less versatile than man-made dichroics.
The last method for polarizing light is called scattering. It occurs when light enters a system of many particles that can absorb energy and re-emit it. Many gases, including air, scatter light. As a photon of light strikes a gas molecule it will excite the molecule, causing it to vibrate. When the molecule rereleases the photon it, too, vibrates along the same plane. If many gas molecules are caused to vibrate along one plane, the scattered light will be, to some degree, polarized. Polarization by scattering is primarily a natural phenomenon and has found almost no use in industry.
While instrumental in the study of light, polarization has many practical uses as well. For example, polarized sunglasses have been developed; in addition to blocking direct sunlight, they also absorb the polarized light that is reflected from ground, water, or snow.
Certain plastic and crystalline materials have the unusual effect of twisting polarized light. With such materials, polarized light enters vibrating along one plane; upon exiting, however, it vibrates along a completely different plane, while still remaining polarized. These materials are said to display optical activity, and are rapidly gaining popularity among engineers--for example, liquid crystal displays (LCDs) rely upon polarization by optically active liquid crystals.
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