Infrared radiation is used in industrial, scientific, military, commercial, and medical applications.

Infrared spectroscopy examines absorption and transmission of photons in the infrared range. It excites vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared radiation is emitted or absorbed by molecules when changing rotational-vibrational movements. The balance between absorbed and emitted infrared radiation has an important effect on Earth's climate. Slightly more than half of the energy from the Sun was eventually found, through Herschel's studies, to arrive on Earth in the form of infrared. In 1800 the astronomer Sir William Herschel discovered that infrared radiation is a type of invisible radiation in the spectrum lower in energy than red light, by means of its effect on a thermometer.

It was long known that fires emit invisible heat in 1681 the pioneering experimenter Edme Mariotte showed that glass, though transparent to sunlight, obstructed radiant heat. As a form of electromagnetic radiation, IR propagates energy and momentum, exerts radiation pressure, and has properties corresponding to both those of a wave and of a particle, the photon. Almost all black-body radiation from objects near room temperature is at infrared wavelengths. Longer IR wavelengths (30 μm-100 μm) are sometimes included as part of the terahertz radiation range. IR is commonly divided between longer wavelength thermal infrared that is emitted from terrestrial sources and shorter wavelength near-infrared that is part of the solar spectrum. IR is generally understood to encompass wavelengths from around 1 millimeter (300 GHz) to the nominal red edge of the visible spectrum, around 700 nanometers (430 THz). It is therefore invisible to the human eye. Infrared (sometimes called infrared light and IR) is electromagnetic radiation (EMR) with wavelengths longer than those of visible light and shorter than radio waves. This false-color infrared space telescope image has blue, green and red corresponding to 3.4, 4.6, and 12 μm wavelengths, respectively.

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