Infrared Radiation
In 1666 Isaac Newton used a prism to disperse a beam of sunlight, splitting the white light into a colored band, or the color spectrum. It consisted of the colors of the rainbow: red, orange, yellow, green, blue, indigo, and violet. For more than a century, scientists believed that the visible spectrum comprised all of the radiation emitted by the Sun. However, in 1800 the German astronomer William Herschel discovered the presence of radiation beyond the red end of the spectrum, which he called infrared light. Herschel's experiment was quite simple and easily repeated. He too used a prism to split sunlight into its component colors, projecting a spectrum upon a screen. He then passed a thermometer through the spectrum, beginning at the blue end and moving slowly toward the red, in order to determine which color transmitted the most heat energy. Herschel noted that the temperature rose as the thermometer neared the red end. He also discovered that the greatest temperature increase occurred past the red, beyond the band of the visible spectrum. He concluded that some invisible radiation must exist beyond red light.
It is now known that infrared light lies between red light and microwaves on the electromagnetic spectrum, with wavelengths ranging from 0.8 to 1000 micrometers (or microns). This is the range at which heat is radiated, and so infrared radiation is sometimes called heat wave radiation. For this reason, infrared generators are often used as heating elements. However, the most important use of infrared radiation has been its role in detection. Devices sensitive to infrared light can be used to "see" in the dark--an ability highly valued by the military.
Since World War II, military engineers have been perfecting infrared detectors. Since they only "see" objects that give off heat, they are often used as "snoopers" to display a group of people or an encampment against a cooler background. Further research has shown that room-temperature objects (including people) radiate infrared light at almost precisely 10 microns. By designing a detector sensitive to 10 micron radiation, one can obtain a perfect heat-illuminated picture of a totally dark room. Military engineers have also equipped missiles with "heat-seeking" infrared guidance systems, that are capable of chasing heat-emitting targets.
Infrared light has proven to be an excellent medium for high-altitude photography. Because of its longer wavelength, infrared radiation can easily penetrate clouds, dust, and haze. A satellite will then take an infrared picture using false color film that is sensitive to red light; this film shifts the color spectrum slightly so that green objects look blue, red objects look green, and infrared sources look red. Infrared photography can identify land masses and bodies of water, as well as areas of vegetation and human population.
Similar heat-detection devices, called thermographs, are used in medicine. By looking at an infrared picture of a patient, the physician can see areas where blood flow is abnormally high or low. This can be a reliable, nonintrusive method for diagnosing certain ailments.
Perhaps the most exciting application of infrared technology in recent years is the field of infrared astronomy. Astronomers became interested in the possibility of constructing an infrared telescope as early as 1950; however, this technology was not realized until 1983, when the United States, Great Britain, and the Netherlands teamed to launch the Infrared Astronomy Satellite (IRAS).
IRAS orbited the Earth at a distance of 600 miles (900 km), carrying an array of 62 helium-cooled infrared detectors. Its job was to map the celestial sky in the infrared range, searching for objects that were too faint to be seen in the visible spectrum. During its operation IRAS discovered the dust trails of several comets, a new variety of galaxy (including one more powerful than two trillion suns), and a strange bulge near the center of the Milky Way. After ten months in orbit IRAS's supply of helium ran out and the project was terminated. While in orbit it had examined almost 95 percent of the known heavens, identifying more than 250,000 different infrared objects. Construction is planned for a new satellite, the Space Infrared Telescope Facility (SIRTIF), to resume where IRAS left off.
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