What Is Infrared?
Infrared (IR) is a form of light (or radiation) that is invisible to the human eye. It is the portion of the electromagnetic spectrum found between visible light and radio waves, at wavelengths extending from 1 to 200 microns. [A micron is one-millionth of a meter, and is abbreviated as µm.]
Astronomers often refer to different types of infrared light, although the precise wavelength ranges are somewhat arbitrary. For the most part, infrared light is thermal radiation; that is, it is a measure of heat. Cooler objects emit IR light at longer wavelengths.