Infrared is invisible radiant energy, electromagnetic radiation with longer wavelengths than those of visible light.
Infrared radiation was discovered in 1800 by the astronomer Sir William Herschel, who discovered a type of invisible radiation in the spectrum lower in energy than red light. He discovered this by noticing its effect on a thermometer.
Infrared radiation is used in industrial, scientific, and medical applications. For example, night-vision devices using active near-infrared illumination allow things to be observed without the observer being detected.
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