Infrared Radiation, electromagnetic radiation with wavelengths between those of visible red light and radio waves. Infrared waves are given off by all warm objects and produce heat in all objects they strike. The waves cause heat by exciting (increasing the movement of) molecules in the substances they strike. The earth is warmed by infrared radiation from the sun.
Infrared lamps (incandescent lamps that emit mostly infrared radiation) are used for drying paint, wallpaper, and printing ink; for keeping food warm in restaurant kitchens; for providing warmth in outdoor waiting areas during cold weather; and for therapeutic heat treatments. Certain kinds of lasers also produce infrared radiation; a beam of infrared radiation from such a laser is used in compact-disc players to read the information stored on the disc.
Infrared radiation can be detected by a variety of electronic devices and by certain kinds of photographic film. Most infrared detectors operate at very low temperatures so that the heat of the detector does not interfere with its readings.
Infrared imaging devices make it possible to obtain images of the infrared radiation reflected or produced by an object. Infrared images have a number of uses; they are used, for example, to check the heat insulation of a building, to identify sources of thermal pollution in bodies of water, and in mapping the distribution of vegetation and other resources on the earth's surface.
In chemistry, the infrared radiation emitted or absorbed by a substance is studied to help determine the substance's molecular structure and composition. Infrared images are used in medicine to detect abnormally warm areas of the body, revealing the location of diseased tissue. In the military, infrared imaging devices are used as an aid in locating enemy troops at night, and devices that detect infrared radiation emitted by aircraft are used to guide certain antiaircraft missiles.
William Herschel, an English astronomer, discovered infrared radiation in 1800. By measuring the temperature of sunlight that was divided into a spectrum, Herschel found that the highest temperatures occurred beyond the red end of the visible spectrum. He named the invisible radiation that caused this heat infrared (meaning "below red") because, having a greater wavelength than visible red light, it has a lower frequency.