Center for Astrophysical Research in Antarctica

Light beyond the visible

Visible light is what you are probably most familiar with -- red, orange, yellow, green, blue, indigo, and violet. But there is light that is "redder than red" and more purple than purple. All the kinds of light, from the "purplest" to the "reddest" are listed below. All these kinds of light together are called the electromagnetic spectrum.

Notes on notation:
Light can be thought of as a wave, and astronomers generally refer to light by its wavelength, or the distance between two peaks of the light wave. The wavelengths of different kinds of light are listed below. The notation that is used for the numbers is called scientific notation: 10 means 10*10*10 = 1000, or 1 with 3 zeros after it. Similarly, 10 means 10*10*10*10*10 = 100000, 1 with 5 zeros after it. Negative exponents mean "move the decimal the other way"... 10 means 1/10 * 1/10 = 0.01 and 10 means 1/10 * 1/10 * 1/10 * 1/10 * 1/10 * 1/10 = 0.000001. Throughout the below, standard metric abbreviations are used: mm = millimeters, cm = centimeters, and km = kilometers.

Gamma Rays
Wavelength = about the size of the nucleus of an atom = 10 to 10 cm. This is the highest energy light -- the "purplest" of all the kinds of light. Gamma rays coming from space only make it through the atmosphere down to about 30-40 km above the Earth's surface, so they must be observed from space. One gamma-ray telescope currently in use is the Compton Gamma Ray Observatory (CGRO), a NASA satellite.
X-rays
Wavelength = about the size of an atom = 10 cm. You may be familiar with x-rays used by your doctor or dentist. X-rays coming from space only make it through the atmosphere down to about 50-60 km above the Earth's surface, so this is another wavelength that must be observed with satellites. ROSAT is a satellite that observes in x-rays, launched by the European Space Agency. The Advanced X-Ray Astrophysics Facility (AXAF) is a satellite that NASA plans to launch in 1998.
Ultraviolet
Wavelength = about the size of a molecule = 10 cm. Ultraviolet light is also called "UV light" and is just beyond the purple of visible light. It is also what gives you a burn when you stay out in the Sun too long. (Since the wavelength of UV light is about the size of molecules, when it hits a molecule just right, it breaks the molecule. When you get lots of broken molecules, your skin hurts.) UV light coming from space doesn't get through very much of the atmosphere at all, fortunately - only to about 130 km above the Earth's surface. (The ozone layer protects us from much of the UV light from the Sun.) UV light also must be observed from space -- the Ultraviolet Imaging Telescope (UIT) flies on board the space shuttle. The Hubble Space Telescope (HST) sees some UV light. (There are other UV telescopes as well.)
Visible
Wavelength = larger than a molecule = 10 cm. Good old visible light is what everyone is familiar with. It, of course, makes it through the atmosphere all the way down to the ground. There are many, many ground-based telescopes that see in the visible. As an example, Yerkes Observatory is an observatory in Wisconsin. The Hubble Space Telescope (HST) also makes observations in the visible.
Infrared
Wavelength = from about the size of the particles in smoke to a millimeter = 10 - 0.1 cm. Infrared light is just past the red of visible light. We sense infrared light (also called IR light) as heat. Water blocks IR light rather well -- a cloud moving over the Sun makes it a lot cooler for us on the ground. IR light from space gets down to about 20-30 km above the ground. IR observations have to be made from telescopes in high, dry places, away from clouds that block the IR. Telescopes high up on Mauna Kea, Hawaii or airborne on the Kuiper Airborne Observatory (KAO) see in the infrared... So do CARA's telescopes at the South Pole.
More on Infrared and light in general from CARA.
More on Infrared astronomy from CARA.

Microwaves
Wavelength = about baked-potato size = 0.1 cm to 10 cm Microwaves are the kind of light generated by your microwave at home that heats up your food. (Microwaves are sometimes classified as part of the radio spectrum.) They also carry cellular phone transmissions. Microwaves from space make it all the way down to the ground. The Cosmic Background Explorer is a satellite that sees microwaves; microwaves are also observed from the South Pole.