Black body radiation, originally described by Gustav Kirchhoff in 1860, consists of light and other electromagnetic radiation that is emitted by a material object that is not reflective. The frequency and intensity of the radiation depends upon the temperature of the body. As anyone who has heated metal with an oxy-acetylene torch is aware, the body, as it is heated, begins to acquire a dull red cast. As the temperature rises, it assumes successively brighter colorations – orange, yellow and eventually a brilliant blue-white.
Black holes were once thought to be perfect black bodies because they absorb all the radiation that comes their way. Now it is known that they emit black-body (Hawking) radiation.
Black-body radiation — including the light from an energized incandescent filament, a burning candle or wood fire — is non-polarized. It consists of a mix of frequencies. The waveforms are oriented every which way in random fashion.
The vibrations of polarized light happen in a single configuration. Like all electromagnetic radiation, light consists of two fields, magnetic and electrical. They oscillate at right angles with respect to one another. Light propagates as a transverse wave, which means the two fields both oscillate perpendicular to the direction of travel.
Most optical materials, such as glass, are known as isotropic. They do not affect the polarization of light traveling through them. A colored glass filter may pass only a single frequency (of varying bandwidth), but this light is still not coherent, i.e. it is not polarized much less collimated.
It is possible to make an optically active material that will affect the polarization of the light that passes through it. A polarized filter is manufactured by means of a chemical process. Long-chain molecules are aligned within the filter, as much as possible, in the same direction. They may be thought of as very thin dark lines engraved on the surface of the glass, so only the light waves of a particular polarization can pass through.
The two lenses from a pair of polarized sunglasses can be separated and stacked so light passing through both can be viewed. It will be seen that as one lens rotates a quarter turn with respect to the other, the light is filtered in varying amounts. If the “engraved lines” are made perpendicular, the pair of lenses becomes opaque. That is how adjustable camera filters work. The same thing can be accomplished electronically. This effect is how a liquid crystal display (LCD) flat-screen TV or oscilloscope works. (Note that plasma screens do not work this way.)
Besides light, other types of electromagnetic radiation may be polarized. The polarization may rotate either clockwise or counter-clockwise, its speed a function of the frequency. This behavior is exploited extensively in antenna design and multiplexing.
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