Infrared radiation, or simply infrared or IR, is electromagnetic radiation (EMR) with longer wavelengths than those of visible light, and is therefore invisible, although it is sometimes loosely called infrared light. It extends from the nominal red edge of the visible spectrum at 700 nanometers (frequency 430 THz), to 1 millimeter (300 GHz). Most of the thermal radiation emitted by objects near room temperature is infrared. Like all EMR, IR carries radiant energy, and behaves both like a wave and like its quantum particle, the photon.

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A dog in Infrared. Photo from Wikipedia

Basically, Infrared shows hot and cold, like the picture on the right. Infrared is sometimes called Thermal Radiation. It is measured in NIR (Near-Infrared), MIR (Mid-Infrared) and FIR (Far-Infrared)