Beyond the Radio Spectrum Help (page 2)
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.
The visible portion of the EM spectrum lies within the wavelength range of 770 to 390 nm. Emissions at the longest wavelengths appear red; as the wavelength decreases, we see orange, yellow, green, blue, indigo, and violet in that order.
Visible light is transmitted fairly well through the atmosphere at all wavelengths. Scattering increases toward the blue, indigo, and violet end of the spectrum. This is why the sky appears blue during the daytime. Long-wavelength light is scattered the least; this is why the Sun often appears red or orange when it is on the horizon. Red is the preferred color for terrestrial line-of-sight laser communications systems for this same reason. Rain, snow, fog, smoke, and dust interfere with the transmission of visible light through the air. We’ll take a detailed look at the characteristics and behavior of visible light in the next chapter.
As the wavelength of an EM disturbance becomes shorter than the smallest we can see, the energy contained in each individual photon increases. The UV range of wavelengths starts at about 390 nm and extends down to about 1 nm. At a wavelength of approximately 290 nm, the atmosphere becomes highly absorptive, and at wavelengths shorter than this, the air is essentially opaque. This protects the environment against damaging ultraviolet radiation from the Sun. Ozone (molecules consisting of three oxygen atoms) in the upper atmosphere is primarily responsible for this effect. Ozone pollution, prevalent in large cities during the summer months, further attenuates UV.
Ordinary glass is virtually opaque at UV, so cameras with glass lenses cannot be used to take photographs in this part of the spectrum. Instead, a pinhole-type device is used, and this severely limits the amount of energy that passes into the detector. While a camera lens has a diameter of several millimeters or centimeters, a pinhole is less than a millimeter across. Another type of device that can be used to sense UV and to measure its intensity at various wavelengths is the spectrophotometer . A diffraction grating is used to disperse EM energy into its constituent wavelengths from IR through the visible and into the UV range. By moving the sensing device back and forth, any desired wavelength can be singled out for analysis. The principle of operation of the spectrophotometer is shown Fig. 18-5. At the extremely short-wavelength end of the UV spectrum (hard UV) , radiation counters are sometimes used, similar to the apparatus employed for the detection of x-rays and gamma rays. For photographic purposes, ordinary camera film will work at the longer UV wavelengths (soft UV) . A special film, rather like x-ray film, is necessary to make hard-UV photos.
UV rays possess an interesting property that can be observed using a so-called black light. Most hobby shops sell lamps of this sort. They are cylindrical in shape, and superficially, they can be mistaken for small fluorescent tubes. (The incandescent black light bulbs sold in department stores are not especially good sources of UV.) When subjected to UV, certain substances glow brightly in the visible range. This is known as fluorescence . Art stores sell acrylic paints that are specially tailored to glow in various colors when UV strikes them. The effect in a darkened room can be striking. The phosphor coatings on CRTs fluoresce under UV too. So will certain living organisms, such as scorpions. If you live in the desert, go outside some night with a black light and switch it on. If there are scorpions around, you’ll find out.
Most of the radiation from the Sun occurs in the IR and visible parts of the EM spectrum. If the Sun were a much hotter star, producing more energy in the UV range, life on any earthlike planet in its system would have developed in a different way, if at all. Excessive exposure to UV, even in the relatively small amounts that reach the Earth’s surface on bright days, can, over time, cause skin cancer and eye cataracts. There is evidence to suggest that excessive exposure to UV suppresses the activity of the immune system, rendering people and animals more susceptible to infectious diseases. Some scientists believe that the ozone hole in the upper atmosphere, prevalent in the southern hemisphere, is growing because of increased production and emission of certain chemicals by humankind. If this is the case, and if the problem worsens, we should expect that it will affect the evolution of life on this planet.
The x-ray spectrum consists of EM energy at wavelengths from approximately 1 nm down to 0.01 nm. (Various sources disagree somewhat on the dividing line between the hard-UV and x-ray regions.) Proportionately, the x-ray spectrum is large compared with the visible range.
X-rays were discovered accidentally in 1895 by a physicist named Wilhelm Roentgen during experiments involving electric currents in gases at low pressure. If the current was sufficiently intense, the high-speed electrons produced mysterious radiation when they struck the anode (positively charged electrode) in the tube. The rays were called x-rays because of their behavior, never before witnessed. The rays were able to penetrate barriers opaque to visible light and UV. A phosphor-coated object happened to be in the vicinity of the tube containing the gas, and Roentgen noticed that the phosphor glowed. Subsequent experiments showed that the rays possessed so much penetrating power that they passed through the skin and muscles in the human hand, casting shadows of the bones on a phosphor-coated surface. Photographic film could be exposed in the same way.
Modern x-ray tubes operate by accelerating electrons to high speed and forcing them to strike a heavy metal anode (usually made of tungsten). A simplified functional diagram of an x-ray tube of the sort used by dentists to locate cavities in your teeth is shown in Fig. 18-6.
Fig. 18-6 . Functional diagram of an x-ray tube.
As the wavelengths of x-rays become shorter and shorter, it becomes increasingly difficult to direct and focus them. This is so because of the penetrating power of the short-wavelength rays. A piece of paper with a tiny hole works very well for UV photography; in the x-ray spectrum, the radiation passes right through the paper. Even aluminum foil is relatively transparent to x-rays. However, if x-rays land on a reflecting surface at a nearly grazing angle, and if the reflecting surface is made of suitable material, some degree of focusing can be realized. The shorter the wavelength of the x-rays, the smaller the angle of incidence, measured relative to the surface (not the normal), must be if reflection is to take place. At the shortest x-ray wavelengths, the angle must be smaller than 1° of arc. This grazing reflection effect is shown in Fig. 18-7 a . A rough illustration of how a high-resolution x-ray observing device achieves its focusing is shown in part b . The focusing mirror is tapered in the shape of an elongated paraboloid. As parallel x-rays enter the aperture of the reflector, they strike its inner surface at a grazing angle. The x-rays are brought to a focal point, where a radiation counter or detector can be placed.
Fig. 18-7 . (a) x-rays are reflected from a surface only when they strike at a grazing angle, (b) A functional diagram of an x-ray focusing and observing device.
X-rays cause ionization of living tissue. This effect is cumulative and can result in damage to cells over a period of years. This is why x-ray technicians in doctors’ and dentists’ offices work behind a barrier lined with lead. Otherwise, these personnel would be subjected to dangerous cumulative doses of × radiation. It only takes a few millimeters of lead to block virtually all x-rays. Less dense metals and other solids also can block x-rays, but these must be thicker. The important factor is the amount of mass through which the radiation must pass. Sheer physical displacement also can reduce the intensity of × radiation, which diminishes according to the square of the distance. However, it isn’t practical for most doctors or dentists to work in offices large enough to make this a viable alternative.
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