Color and the Spectrum: Infrared
Steve Beeson, Arizona State University


How do infrared goggles work?



Just below (or just above, depending on which way you look at it) the visible spectrum is infrared (IR) radiation. These are longer wavelength light rays (about 1.00 mm to 8 x 10-7m, or 80,000) with energies between 10-3 eV and 1 eV.

Many molecules tend to resonate or vibrate easily when exposed to IR radiation, so we erroneously think of IR as heat waves. The sun emits about half of its radiation in the IR and even home light bulbs emit more IR than visible light.

Many devices use IR as a means of transmitting information (IR lasers, TV remote controls) and of receiving signals (IR telescopes, photographic films, satellites). In order to "see" at night, IR goggles pick up the weak infrared radiation from our warm bodies, translating the signal into visible light that our eyes can detect.

Astronomers use IR telescopes to see inside and beyond huge clouds of gas and dust that block our (visible light) view of such places as the center of the Galaxy. Art preservationists use IR cameras to view paintings that might have underdrawings not visible with the human eye. Charcoal absorbs IR quite well, but most pigments of paint are transparent to infrared. If one shines an IR light on a painting with an underdrawing made of charcoal, the paint layers will pass the radiation, the charcoal will absorb the IR, and the camera will see the white background that reflected the radiation with dark areas corresponding to the drawing made with a charcoal pencil.


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Steve Beeson, Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287