Infrared
In 1800, the astronomer William Herschel conducted an experiment that changed how humanity understood heat. He used a glass prism to refract sunlight and placed thermometers in the resulting spectrum. The mercury rose highest not within the visible colors but just beyond the red light where no color appeared. Herschel called these invisible waves "Calorific Rays" because they carried more heat than any other part of the sun's energy reaching Earth. This discovery proved that invisible radiation existed alongside the light humans could see. It was published before the Royal Society of London later that same year.
For decades after this event, scientists struggled to name and categorize what Herschel had found. The word infrared did not appear until late in the nineteenth century when French physicist Edmond Becquerel coined the term infra-red in 1867. By 1874, English translations of articles by Vignaud Dupuy de Saint-Florent brought the concept into common usage. Earlier experiments by Marc-Auguste Pictet in 1790 had already demonstrated the reflection of radiant heat using mirrors without visible light. These early pioneers laid the groundwork for understanding thermal radiation as a distinct form of electromagnetic energy.
No universally accepted definition exists for the exact range of infrared radiation across all scientific fields. Typically, the band extends from the nominal red edge of the visible spectrum at 780 nanometers up to one millimeter. This wavelength range corresponds to frequencies between approximately 430 terahertz down to 300 gigahertz. Beyond this limit lies the microwave portion of the electromagnetic spectrum. Increasingly, terahertz radiation is counted as part of the microwave band rather than infrared, moving the boundary to 0.1 millimeters or 3 terahertz.
Different organizations create their own subdivision schemes based on specific applications and detector technologies. The International Commission on Illumination recommends three bands labeled IR-A, IR-B, and IR-C. ISO 20473 specifies Near infrared from 0.78 micrometers to 3 micrometers, Mid infrared from 3 to 50 micrometers, and Far infrared from 50 to 1,000 micrometers. Astronomers use capital letters like I, J, H, and K for near-infrared wavelengths while L, M, N, and Q refer to mid-infrared regions. These divisions vary depending on whether the focus is atmospheric windows, water absorption, or sensor response capabilities.
Sunlight arriving at Earth contains slightly more than half its energy in the form of infrared radiation. At sea level during zenith, sunlight provides an irradiance of just over one kilowatt per square meter. Of this total energy, 527 watts are infrared radiation while 445 watts constitute visible light and only 32 watts represent ultraviolet radiation. Nearly all the infrared radiation in sunlight falls within the near-infrared band shorter than four micrometers. This balance between absorbed and emitted infrared radiation has a profound effect on Earth's climate systems.
Objects at room temperature emit radiation concentrated mostly in the eight to twenty-five micrometer band. Black-body radiation is continuous and radiates at all wavelengths associated with molecular vibration. Fires produce far more infrared energy than visible-light energy despite being hot enough to glow. Lightning and natural fires are among the few processes hot enough to produce much visible energy alongside their thermal output. The concept of emissivity determines how surfaces deviate from ideal black body behavior when emitting heat.
In 1945, the Zielgerät 1229 Vampir system became the first portable infrared device introduced for military applications. By 1958, Falcon and Sidewinder missiles were developed using infrared technology to track targets. These weapons function as passive guidance systems that use infrared signatures emitted by jet engine exhaust plumes. The three to five micrometer portion of the spectrum serves as an atmospheric window where seekers operate effectively. Humans at normal body temperature radiate chiefly at wavelengths around ten micrometers which allows detection by specialized sensors.
Night vision devices convert ambient light photons into electrons through chemical and electrical amplification before converting them back into visible light. Active-infrared illumination allows observers to see without emitting detectable visible light sources. Infrared tracking enables homing on vehicles or aircraft based on contrast between warm engines and cooler backgrounds. Thermal imaging cameras detect radiation in the nine to fourteen micrometer range to create images showing surface temperatures. This technology helps firefighting crews locate people inside smoke-filled buildings and assists soldiers in identifying enemies during darkness.
Infrared vibrational spectroscopy identifies molecules by analyzing their constituent bonds vibrating at characteristic frequencies. A group of atoms like CH2 may have multiple oscillation modes caused by stretching and bending motions. If an oscillation leads to a change in dipole moment within the molecule it will absorb a photon matching that frequency. Scientists typically study organic compounds using light from the mid-infrared band ranging from four thousand to four hundred inverse centimeters. This technique reveals sample composition in terms of chemical groups present and indicates purity levels.
Astronomers observe distant objects using optical components including mirrors lenses and solid state digital detectors cooled with liquid helium. Cold dark molecular clouds glow with radiated heat when irradiated by embedded stars allowing detection of protostars before they emit visible light. Distant galaxies with high redshift shift their peak spectrum toward longer wavelengths making them more readily observed in infrared. Water vapor in Earth's atmosphere absorbs portions of incoming infrared radiation limiting ground-based telescope sensitivity. Space telescopes avoid this handicap entirely since outer space contains no atmospheric interference to block signals.
Weather satellites equipped with scanning radiometers produce thermal images showing cloud heights and types across the globe. The scanning typically occurs in the ten point three to twelve point five micrometer range known as IR4 and IR5 channels. High cold tops such as cyclones appear red or black while lower warmer stratus clouds display blue or grey tones. Hot land surfaces show up as dark-grey or black against the background imagery. These pictures allow analysts to locate ocean surface features like eddies or vortices that map currents such as the Gulf Stream.
Infrared cleaning techniques reduce dust and scratches on motion picture film scanners by collecting an additional channel at the same resolution as visible color data. Art conservators use reflectography to reveal underlying layers in paintings including artist underdrawings called pentimenti. Carbon black ink shows up extremely well in these scans allowing examination of documents like the Dead Sea Scrolls or Roman works from Villa of the Papyri. Remote temperature sensing helps farmers protect crops against frost while fishermen increase catches by monitoring sea temperatures. Short-range wireless communication uses infrared light-emitting diodes modulated according to codes receivers interpret for device control.
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Common questions
When did William Herschel discover infrared radiation?
William Herschel discovered infrared radiation in 1800 when he conducted an experiment using a glass prism to refract sunlight and measured the temperature of different colors. He found that the mercury rose highest just beyond the red light where no color appeared, proving invisible heat waves existed.
Who coined the term infrared and when was it first used?
French physicist Edmond Becquerel coined the term infra-red in 1867 after decades of struggle by scientists to name and categorize what Herschel had found. English translations of articles by Vignaud Dupuy de Saint-Florent brought the concept into common usage by 1874.
What is the wavelength range of infrared radiation?
The band typically extends from the nominal red edge of the visible spectrum at 780 nanometers up to one millimeter. This wavelength range corresponds to frequencies between approximately 430 terahertz down to 300 gigahertz before transitioning into the microwave portion of the electromagnetic spectrum.
How much energy does infrared radiation contribute to sunlight reaching Earth?
Sunlight arriving at Earth contains slightly more than half its energy in the form of infrared radiation with 527 watts out of over one kilowatt per square meter being infrared. Nearly all the infrared radiation in sunlight falls within the near-infrared band shorter than four micrometers.
When did military forces first use portable infrared devices?
In 1945, the Zielgerät 1229 Vampir system became the first portable infrared device introduced for military applications. By 1958, Falcon and Sidewinder missiles were developed using infrared technology to track targets through passive guidance systems that use infrared signatures emitted by jet engine exhaust plumes.