image of two people taken in long-wavelength infrared (body-temperature thermal) light.
This infrared space telescope image has (false color) blue, green and red corresponding to 3.4 4.6, and 12
Infrared radiation is
electromagnetic radiation (EMR) with longer
wavelengths than those of
visible light, and is therefore invisible to the human eye, although it is sometimes loosely called infrared light. It extends from the nominal
red edge of the
visible spectrum at 700
THz), to 1
 (although specially pulsed lasers can allow humans to detect IR radiation up to 1050 nm.
). 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
Infrared was discovered in 1800 by astronomer Sir
William Herschel, who discovered a type of invisible radiation in the spectrum lower in energy than red light, by means of its effect on a thermometer.
 Slightly more than half of the total energy from the Sun was eventually found to arrive on Earth in the form of infrared. The balance between absorbed and emitted infrared radiation has a critical effect on Earth's
Infrared radiation is emitted or absorbed by
molecules when they change their
rotational-vibrational movements. 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 spectroscopy examines absorption and transmission of
photons in the infrared range.
Infrared radiation is used in industrial, scientific, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected.
Infrared astronomy uses sensor-equipped
telescopes to penetrate dusty regions of space such as
molecular clouds, detect objects such as
planets, and to view highly
red-shifted objects from the early days of the
 Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect overheating of electrical apparatus.
Thermal-infrared imaging is used extensively for military and civilian purposes. Military applications include
target acquisition, surveillance,
night vision, homing, and tracking. Humans at normal body temperature radiate chiefly at wavelengths around 10 μm (micrometers). Non-military uses include
thermal efficiency analysis, environmental monitoring, industrial facility inspections, remote temperature sensing, short-ranged