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
means 10*10*10 = 1000, or 1 with 3 zeros after it.
means 10*10*10*10*10 = 100000, 1 with 5 zeros after
it. Negative exponents mean "move the decimal the other way"...
means 1/10 * 1/10 = 0.01 and
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.
More on Infrared and light in general from
the perspective of CARA.
- Wavelength = about the size of the nucleus of an atom =
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
Ray Observatory (CGRO), a NASA satellite.
- Wavelength = about the size of an atom =
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.
X-Ray Astrophysics Facility (AXAF) is a satellite that NASA plans
to launch in 1998.
- Wavelength = about the size of a molecule =
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
Imaging Telescope (UIT) flies on board the space shuttle.
The Hubble Space
Telescope (HST) sees some UV light. (There are other UV telescopes
- Wavelength = larger than a molecule =
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
Telescope (HST) also makes observations in the visible.
- Wavelength = from about the size of the particles in smoke to a
- 0.1 cm. Infrared light is just past the red of
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
Observatory (KAO) see in the infrared... So do CARA's telescopes at
the South Pole.
More on Infrared and light in general from
More on Infrared astronomy from CARA.
- 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.
Background Explorer is a satellite that sees microwaves;
microwaves are also observed from the South Pole.
- Wavelength = from a tv antenna to beyond the size of a building =
10 cm to beyond
This part of the spectrum is as red as you can get. Radio waves carry
radio and television broadcasts. Radio waves from space mostly make
it all the way down to the ground.
Radio telescopes have to be big enough to catch those waves; they're
generally made up of lots of smaller "eyes", like those at the
Large Array (VLA).
More on Infrared astronomy from the
perspective of CARA.