Beyond the Radio Spectrum Help
The shortest rf waves measure approximately 1 mm; this corresponds to a frequency of 300 GHz. As the wavelength becomes shorter than this, we encounter the IR, visible, UV, x-ray, and gamma-ray spectra in that order.
The longest IR waves are approximately 1 mm in length; the reddest visible light has a wavelength of a little less than 0.001 mm. This is a span of a thousandfold, or three mathematical orders of magnitude. In terms of frequency, the IR spectrum lies below the visible red spectrum, and it is from this fact that it gets its name ( infra - means “below”). Our bodies sense IR radiation as warmth or heat. The IR rays are not literally heat, but they produce heat when they strike an absorptive object such as the human body.
The Sun is a brilliant source of IR; it emits just about as much IR as visible light. Other sources of IR include incandescent light bulbs, fire, and electrical heating elements. If you have an electric stove and switch on one of the burners to low, you can feel the IR radiation from it even though the element appears black to the eye.
Infrared radiation can be detected by special films that can be used in most ordinary cameras. Some high-end photographic cameras have focus numbers for IR as well as for visible light printed on their lens controls. Glass transmits IR at the shorter wavelengths (near IR) but blocks IR at the longer wavelengths (far IR) . When you take an IR photograph in visible-light darkness, warm objects show up clearly. This is the principle by which some night-vision apparatus works. Infrared-detecting equipment has been used recently in wartime to detect the presence and movement of personnel.
The fact that glass transmits near IR but blocks far IR is responsible for the ability of glass greenhouses to maintain interior temperatures much higher than that of the external environment. It is also responsible for the extreme heating of automobile interiors on sunny days when the windows are closed. This effect can be used to advantage in energy-efficient homes and office buildings. Large windows with southern exposures can be equipped with blinds that are opened on sunny winter days and closed in cloudy weather and at night.
IR radiation at low and moderate levels is not dangerous and in fact has been used therapeutically to help relieve the discomfort of joint injuries and muscle strains. At high intensity, however, IR radiation can cause burns. In massive structural or forest fires, this radiation can scorch the clothing off a person and then literally cook the body alive. The most extreme earthly IR radiation is produced by the detonation of a nuclear bomb or by an asteroid impact. The IR burst from a 20-megaton weapon (equivalent to 2 × 10 7 tons of conventional explosive) can kill every exposed living organism within a radius of several kilometers.
In some portions of the IR spectrum, the atmosphere of our planet is opaque. In the near IR between about 770 nm (the visible red) and 2,000 nm, our atmosphere is reasonably clear. Water vapor causes attenuation in the IR between the wavelengths of approximately 4,500 and 8,000 nm. Carbon dioxide (CO 2 ) gas interferes with the transmission of IR at wavelengths ranging from about 14,000 to 16,000 nm. Rain, snow, fog, and dust interfere with the propagation of IR. The presence of CO 2 in the atmosphere keeps the surface warmer than it would be if there were less CO 2 . Most scientists agree that increasing CO 2 in the atmosphere will produce a significant rise in the average surface temperature. This greenhouse effect gets its name from the fact that the CO 2 in the Earth’s atmosphere treats IR in much the same way as the glass in a greenhouse.
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