Infrared is best known as being heat radiation. The shortest infrared rays are very close to being visible to humans, hence the name (Latin infra meaning below, "below red"). However, humans can sense this radiation as heat, and infrared makes up a majority of the radiation that the Earth receives at ground level. Animals absorb heat and emit it themselves, and these phenomena can be captured by thermographic images:
A thermographic image showing two people (false-color).
Infrared radiation is subdivided further into three zones, near-infrared (.7 to 5 microns, or .0000007 to .000005 meters), mid-infrared (5 microns to about 35 microns), and far-infrared (35 microns to about 300 microns). Each zone has separate uses, notably near-infrared for telecommunication, mid-infrared for heat-seeking missiles, and far-infrared for lasers.
The zones are labeled in this way because near-infrared is on the cusp of visible light, while far-infrared has a much longer wavelength. Some living things can naturally "see" into the infrared spectrum, such as some snakes that use a special heat-detecting sense to find live prey.
Infrared rays have many other uses, including night vision and the heating of objects. Infrared cameras are also a valuable resource in meteorology, as the surface of the earth emits more infrared radiation than that of clouds, creating an easy way to analyze cloud height and temperature.
An infrared image of the Atlantic Basin. Clouds of increasing height are brighter white. The image also shows Tropical Storm Richard in the Caribbean. Image taken 10/23/10 12:15 UTC.
Infrared astronomy was pioneered by the man who discovered infrared rays: William Herschel. He is also well known for discovering the planet Uranus, along with two of its moons. His discovery was made while studying the spectrum of visible light produced by a prism. He noted that there was a significant temperature increase outside the visible light spectrum, beyond the red. He performed more tests and concluded that it was indeed a new type of radiation, being absorbed and emitted just like visible light, and also being released in massive quantities from the Sun.
In modern times, near-infrared rays can usually be picked up with a normal optical telescope, as these telescopes often have larger visual ranges than the naked eye, seeing into both near-infrared and near-ultraviolet. As one progresses farther into the infrared spectrum, a vast majority of rays do not reach the surface of the earth. However, telescopes at high altitudes in dry environments can pick these rays up with good efficiency. Telescopes operating in infrared can detect objects behind interstellar dust clouds and within nebulas better than visible light, as they can see the heat emitted from the region.
A photo of the Orion constellation in visible (left) and infrared (right). Although the infrared provides little indication to the exact location of the stars, it detects gas clouds throughout the constellation and other features totally invisible in the optical spectrum.
Overall, infrared rays have important uses, such as telecommunication and lasers, as well as transmitting heat throughout the Universe.
Sources: National Hurricane Center, http://www.ipac.caltech.edu/Outreach/Edu/Regions/irregions.html, http://en.wikipedia.org/wiki/Infrared. http://www.nasa.gov/mission_pages/SOFIA/infrared.html
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