What Is Infrared?
An image of Earth in infrared wavelengths shows relative temperatures around the world. The photo includes a plume of carbon monoxide pollution near the Rim Fire that burned near Yosemite National Park in California on Aug. 26, 2013.
Credit: NASA/JPL-Caltech/Space Science Institute

Infrared radiation (IR), or infrared light, is a type of radiant energy that's invisible to human eyes but that we can feel as heat. All objects in the universe emit some level of IR radiation, but two of the most obvious sources are the sun and fire.

IR is a type of electromagnetic radiation, a continuum of frequencies produced when atoms absorb and then release energy. From highest to lowest frequency, electromagnetic radiation includes gamma-rays, X-rays, ultraviolet radiation, visible light, infrared radiation, microwaves and radio waves. Together, these types of radiation make up the electromagnetic spectrum.

British astronomer William Herschel discovered infrared light in 1800, according to NASA. In an experiment to measure the difference in temperature between the colors in the visible spectrum, he placed thermometers in the path of light within each color of the visible spectrum. He observed an increase in temperature from blue to red, and he found an even warmer temperature measurement just beyond the red end of the visible spectrum.

Within the electromagnetic spectrum, infrared waves occur at frequencies above those of microwaves and just below those of red visible light, hence the name "infrared." Waves of infrared radiation are longer than those of visible light, according to the California Institute of Technology (Caltech). IR frequencies range from about 3 gigahertz (GHz) up to about 400 terahertz (THz), and wavelengths are estimated to range between 1,000 micrometers (µm) and 760 nanometers (2.9921 inches), although these values are not definitive, according to NASA.

Similar to the visible light spectrum, which ranges from violet (the shortest visible-light wavelength) to red (longest wavelength), infrared radiation has its own range of wavelengths. The shorter "near-infrared" waves, which are closer to visible light on the electromagnetic spectrum, don't emit any detectable heat and are what's discharged from a TV remote control to change the channels. The longer "far-infrared" waves, which are closer to the microwave section on the electromagnetic spectrum, can be felt as intense heat, such as the heat from sunlight or fire, according to NASA.

IR radiation is one of the three ways heat is transferred from one place to another, the other two being convection and conduction. Everything with a temperature above around 5 degrees Kelvin (minus 450 degrees Fahrenheit or minus 268 degrees Celsius) emits IR radiation. The sun gives off half of its total energy as IR, and much of the star's visible light is absorbed and re-emitted as IR, according to the University of Tennessee.

Household appliances such as heat lamps and toasters use IR radiation to transmit heat, as do industrial heaters such as those used for drying and curing materials. Incandescent bulbs convert only about 10 percent of their electrical energy input into visible light energy, while the other 90 percent is converted to infrared radiation, according to the Environmental Protection Agency.

Infrared lasers can be used for point-to-point communications over distances of a few hundred meters or yards. TV remote controls that rely on infrared radiation shoot out pulses of IR energy from a light-emitting diode (LED) to an IR receiver in the TV, according to How Stuff Works. The receiver converts the light pulses to electrical signals that instruct a microprocessor to carry out the programmed command.

One of the most useful applications of the IR spectrum is in sensing and detection. All objects on Earth emit IR radiation in the form of heat. This can be detected by electronic sensors, such as those used in night vision goggles and infrared cameras.

A simple example of such a sensor is the bolometer, which consists of a telescope with a temperature-sensitive resistor, or thermistor, at its focal point, according to the University of California, Berkeley (UCB). If a warm body comes into this instrument's field of view, the heat causes a detectable change in the voltage across the thermistor.

Night vision cameras use a more sophisticated version of a bolometer. These cameras typically contain charge-coupled device (CCD) imaging chips that are sensitive to IR light. The image formed by the CCD can then be reproduced in visible light. These systems can be made small enough to be used in hand-held devices or wearable night-vision goggles. The cameras can also be used for gun sights with or without the addition of an IR laser for targeting.

Infrared spectroscopy measures IR emissions from materials at specific wavelengths. The IR spectrum of a substance will show characteristic dips and peaks as photons (particles of light) are absorbed or emitted by electrons in molecules as the electrons transition between orbits, or energy levels. This spectroscopic information can then be used to identify substances and monitor chemical reactions.

According to Robert Mayanovic, professor of physics at Missouri State University, infrared spectroscopy, such as Fourier transform infrared (FTIR) spectroscopy, is highly useful for numerous scientific applications. These include the study of molecular systems and 2D materials, such as graphene.

Caltech describes infrared astronomy as "the detection and study of the infrared radiation (heat energy) emitted from objects in the universe." Advances in IR CCD imaging systems have allowed for detailed observation of the distribution of IR sources in space, revealing complex structures in nebulas, galaxies and the large-scale structure of the universe.

One of the advantages of IR observation is that it can detect objects that are too cool to emit visible light. This has led to the discovery of previously unknown objects, including comets, asteroids and wispy interstellar dust clouds that seem to be prevalent throughout the galaxy.

IR astronomy is particularly useful for observing cold molecules of gas and for determining the chemical makeup of dust particles in the interstellar medium, said Robert Patterson, professor of astronomy at Missouri State University. These observations are conducted using specialized CCD detectors that are sensitive to IR photons.

Another advantage of IR radiation is that its longer wavelength means it doesn't scatter as much as visible light, according to NASA. Whereas visible light can be absorbed or reflected by gas and dust particles, the longer IR waves simply go around these small obstructions. Because of this property, IR can be used to observe objects whose light is obscured by gas and dust. Such objects include newly forming stars imbedded in nebulas or the center of Earth's galaxy.

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This article was updated on Feb. 27, 2019, by Live Science contributor Traci Pedersen.