Infrared, << ihn fruh REHD, >> rays, also called infrared radiation or thermal radiation, are a form of electromagnetic wave that cannot be seen by the unaided eye. Electromagnetic waves are traveling patterns of electric and magnetic influence. Any warm object, such as an object at room temperature, emits (gives off) infrared radiation due to its heat. If the temperature is increased, the object will emit more infrared rays. If the temperature becomes very high, the object will glow with rays of visible light as well as infrared. For example, an ordinary light bulb’s filament (thin, coiled wire) glows with both kinds of rays when its temperature rises to more than 4500 °F (2500 °C).
Human beings and other warm-blooded creatures emit more thermal radiation than their surroundings and therefore glow brightly with infrared rays. Police use special cameras that can detect infrared radiation to locate people in dark or foggy areas. Firefighters use such cameras to find people in smoke-filled rooms.
Military applications include the detection of heat exhaust from tanks and aircraft at night. Astronomers use infrared-detecting telescopes to search for the thermal radiation emitted by newborn stars hidden inside clouds of gas and dust. Remote controls for television sets and many other electronic devices use infrared radiation. Infrared cameras can be used to measure the temperature of a distant object.
Infrared rays, like rays of light and radio waves, are electromagnetic waves. All these waves travel in a vacuum at the same speed, 186,282 miles (299,792 kilometers) per second. What distinguishes them from one another is their wavelength (distance between successive wave crests). The wavelengths of infrared rays are longer than those of light and shorter than those of radio waves. Infrared wavelengths range from about 700 nanometers to 1 millimeter. One nanometer is one-billionth of a meter, and 1 millimeter is one-thousandth of a meter.
British astronomer Sir William Herschel discovered infrared rays in 1800. He used a prism to break up light from the sun. The prism spread out the different colors of sunlight into a band called a spectrum. At one end of the spectrum was violet light, which has the shortest wavelengths. At the other end was red light, with the longest wavelengths. Herschel then used a thermometer to measure heating effects at different parts of the spectrum—and beyond. He found a strong heating effect beyond the red end of the spectrum. He concluded that invisible rays were producing this effect. Infrared radiation gets its name from its location on the electromagnetic spectrum. The prefix infra- means below.
