A pseudo color 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  µm wavelengths, respectively.

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 nanometers ( frequency 430  THz), to 1  millimeter (300  GHz) [1] (although specially pulsed lasers can allow humans to detect IR radiation up to 1050 nm. [2] [3] [4] [5]). 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.

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. [6] 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 climate.

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. [7]

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 universe. [8] 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 wireless communication, spectroscopy, and weather forecasting.

Definition and relationship to the electromagnetic spectrum

Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nanometers (nm) to 1 mm. This range of wavelengths corresponds to a frequency range of approximately 430  THz down to 300  GHz. Below infrared is the microwave portion of the electromagnetic spectrum.

Infrared in relation to electromagnetic spectrum
Light comparison [9]
Name Wavelength Frequency (Hz) Photon Energy (eV)
Gamma ray less than 0.01 nm more than 30 EHz 124 keV – 300+ GeV
X-ray 0.01 nm – 10 nm 30 EHz – 30 PHz 124 eV  – 124 keV
Ultraviolet 10 nm – 400 nm 30 PHz – 790 THz 3.3 eV – 124 eV
Visible 400 nm–700 nm 790 THz – 430 THz 1.7 eV – 3.3 eV
Infrared 700 nm – 1 mm 430 THz – 300 GHz 1.24 meV – 1.7 eV
Microwave 1 mm – 1 meter 300 GHz – 300 MHz 1.24 µeV – 1.24 meV
Radio 1 meter – 100,000 km 300 MHz3 Hz 12.4 feV – 1.24 µeV
Other Languages
Afrikaans: Infrarooi
Alemannisch: Infrarotstrahlung
aragonés: Infrarroyo
Bân-lâm-gú: Âng-gōa-sòaⁿ
беларуская (тарашкевіца)‎: Інфрачырвонае выпраменьваньне
català: Infraroig
Cymraeg: Is-goch
euskara: Infragorri
فارسی: فروسرخ
Fiji Hindi: Infrared
français: Infrarouge
한국어: 적외선
हिन्दी: अवरक्त
Bahasa Indonesia: Inframerah
interlingua: Infrarubie
íslenska: Innrautt ljós
עברית: תת-אדום
Basa Jawa: Infra abang
Kiswahili: Infraredi
Kreyòl ayisyen: Onn enfrawouj
Кыргызча: Инфракызыл нур
Lëtzebuergesch: Infraroutstralung
Limburgs: Infraroead
Bahasa Melayu: Sinar inframerah
Nederlands: Infrarood
नेपाल भाषा: इन्फ्रारेड
日本語: 赤外線
norsk nynorsk: Infraraud stråling
occitan: Infraroge
oʻzbekcha/ўзбекча: Infraqizil nurlanish
پنجابی: تھلویں لال
polski: Podczerwień
română: Infraroșu
Scots: Infrareid
Simple English: Infrared
slovenščina: Infrardeče valovanje
srpskohrvatski / српскохрватски: Infracrveno zračenje
Basa Sunda: Infrabeureum
Tagalog: Infrared
татарча/tatarça: Инфракызыл нурланыш
Türkçe: Kızılötesi
اردو: زیرسرخ
ئۇيغۇرچە / Uyghurche: ئىنفرا قىزىل نۇر
Tiếng Việt: Tia hồng ngoại
Winaray: Infrared
ייִדיש: אינפרארויט
粵語: 紅外光
中文: 红外线