Infrared Radiation

Infrared radiation is electromagnetic radiation with wavelengths longer than visible red light, from about 700 nanometers to 1 millimeter. People feel it as heat, and it is the main form in which Earth gives off energy to space.

William Herschel discovered infrared in 1800 when he found that a thermometer placed beyond the red end of a prism's spectrum still warmed. The band is divided into near, mid and far infrared. All objects above absolute zero emit it, and the hotter they are the shorter the wavelength at which they emit most strongly, a relationship known as Wien's law. Thermal cameras, night-vision devices, remote controls and many astronomical instruments use it.

Earth absorbs solar radiation mostly as visible light and returns the energy as infrared, centered near a wavelength of 10 micrometers. Molecules such as water vapor, carbon dioxide and methane absorb parts of that outgoing infrared and re-emit it in all directions, which warms the surface. This is the physical basis of the greenhouse effect, and changes in the balance are tracked in Earth's energy budget.

Satellites measure infrared emission to find the temperature of the surface, clouds and atmosphere layers. Infrared spectroscopy identifies molecules by the wavelengths they absorb, and space telescopes observe in infrared to see through dust and to detect very distant galaxies whose light has been stretched to longer wavelengths. Related ideas are covered in earth observation and remote sensing.

• Greenhouse Effect — the warming caused by its absorption
• Ultraviolet Radiation — radiation at the other end of visible light
• Earth's Energy Budget — the balance of incoming and outgoing energy