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Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of radio waves. The name means "below red" (from the Latin infra, "below"), red being the color of visible light.. Infrared radiation spans three orders of magnitude and has wavelengths between approximately 750 nm and 1 mm.[1]

Infrared radiation was discovered in 1800 by astronomer William Herschel, who discovered a type of invisible radiation in the light spectrum beyond red light, by means of its affect upon a thermometer. Slightly more than half of the total energy from the Sun was eventually found to arrive on Earth in the form of infrared.

The infrared portion of the electromagnetic spectrum is usually divided into three regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum. The higher-energy near-IR wavelength can excite overtone or harmonic vibrations.

The far-infrared, lying adjacent to the microwave region, has low energy.


Image of two girls in mid-infrared ("thermal") light (false color)