Infrared Radiation

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What is infrared radiation?

Infrared (IR) radiation is simply one of the many types of 'light' that comprise the electromagnetic (EM) spectrum. Infrared light is characterized by wavelengths that are longer than visible light (4000-7000 Angstroms, or 0.4 0.7 micrometers; also denoted as microns). Astronomers generally divide the infrared portion of the EM spectrum into three regions: near-infrared (0.7 5 microns), mid-infrared (5 30 microns) and far-infrared (30 1000 microns).
Who discovered infrared light, and when did the discovery occur?
The famous astronomer William Herschel, who also discovered the first new planet since antiquity (Uranus) and studied sunspots, was the first to discover a form of light other than visible (optical). In an 1800 experiment, Herschel used a glass prism to spread sunlight into a rainbow of colors. He then measured the temperature of each color of visible light and noted differences. Most intriguingly, he found a curious reading when the thermometer bulb was placed just beyond the red portion of the visible spectrum. He had discovered thermal radiation, which has come to be known as infrared. [The prefix "infra" means "below."]
Is infrared radiation dangerous?
In general, no -- at least from naturally occurring physical processes. Any form of radiation -- including visible light or radio waves -- could potentially be dangerous if highly concentrated into a narrow beam (that is the principle of lasers) of very high power. We are immersed in infrared radiation everyday. It is nothing more than heat. On the other hand, you certainly would not want to place your hand on a hot stove, in which case IR radiation would be dangerous.
What kinds of objects emit infrared radiation?
All objects that are not at absolute zero emit infrared radiation. Absolute zero defines the temperature where all molecular motion ceases, and is the coldest possible temperature. It corresponds to about minus 273 degrees Celsius, or minus 460 degrees Fahrenheit. [Physicists define this point to be zero degrees Kelvin, with each increment on the Kelvin scale identical to that of the Celsius scale.] Even ice cubes emit thermal heat!
How does our atmosphere block infrared radiation from space?
Only certain parts of the electromagnetic spectrum (all light ranging from gamma ray to radio waves) can make it to the Earth's surface. Much is absorbed by our atmosphere. Visible light, radio waves and a few small ranges of infrared wavelengths do make it through. Gamma rays, and most of the ultraviolet rays and infrared rays do not. Much of the infrared light is absorbed by water vapor in our atmosphere. This is why infrared telescopes are placed on high, dry mountains (like Mauna Kea in Hawaii) so that they can observe more infrared radiation. The only way to study the entire range of infrared (as well as gamma rays, xrays, UV) is to place telescopes in space well above the atmosphere. Only some (not all) of the IR radiation between 1 and 40 microns makes it to the Earth's surface. The rest is absorbed by our atmosphere primarily by water vapor. IR is also absorbed to a lesser degree by carbon dioxide and oxygen molecules.
What materials block, reflect, absorb or emit infrared light?
Thermal infrared, which corresponds to wavelengths longer than about 5 microns is a direct measure of temperature. One simple material that blocks IR is plexiglass. We use it in a demonstration of infrared radiation and the greenhouse effect. Also, water is a good absorber of IR. Hence, one must get above the atmosphere's water vapor to conduct most infrared astronomy measurements. Any good mirror should also be capable of reflecting infrared light. Most ground-based telescopes that observe in the "near-infrared" (betweewn 1 and 2.5 microns) rely on the same telescopes as for optical astronomy. Finally, thermal IR is a measure of heat, and *any* object above absolute zero (-273 C) emits infrared radiation. The hotter a source, the more IR light it emits.
What are the benefits of infrared technology?
There are several advantages to detecting and studying infrared radiation. Infrared is basically heat radiation. Infrared radiation carries information about the temperature distribution of the objects studied. Infrared can also penetrate, thick smoke, clouds and dust. This makes infrared cameras very useful in search and rescue and firefighting. Many lives have been saved by thermal infrared cameras - finding people lost at night or at sea by detecting their body heat, or finding people in a smoke filled building. Infrared is widely used in the sciences in astronomy, meteorology, oceanography and archeology. It is used to inspect mechanical and electrical systems, in animal studies, in medicine, navigation, law enforcement, in the military as well as in food studies.
 
See our web site "Seeing Our World in a Different Light" for more details.

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