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Introduction...
Measuring the
electromagnetic spectrum
You actually know more about it than you may think! The electromagnetic
(EM) spectrum is just a name that scientists give a bunch of types of
radiation when they want to talk about them as a group. Radiation is
energy that travels and spreads out as it goes-- visible light that
comes from a lamp in your house or radio waves that come from a radio
station are two types of electromagnetic radiation. Other examples of EM
radiation are microwaves, infrared and ultraviolet light, X-rays and
gamma-rays. Hotter, more energetic objects and events create higher
energy radiation than cool objects. Only extremely hot objects or
particles moving at very high velocities can create high-energy
radiation like X-rays and gamma-rays.
Here are the different types of radiation in the EM spectrum, in order
from lowest energy to highest:
Radio: yes,
this is the same kind of energy that radio stations emit into the air
for your boom box to capture and turn into your favorite Mozart,
Madonna, or Coolio tunes. But radio waves are also emitted by other
things such as stars and gases in space. You may not be able to
dance to what these objects emit, but you can use it to learn what they
are made of.
 Microwaves: they will cook your popcorn in just a few minutes! In
space, microwaves are used by astronomers to learn about the structure
of nearby galaxies, including our own Milky Way!
Infrared: we often think of this as being the same thing as 'heat',
because it makes our skin feel warm. In space, IR light maps the dust
between stars.
Visible: yes, this is the part that our eyes see. Visible radiation is
emitted by everything from fireflies to light bulbs to stars ... also by
fast-moving particles hitting other particles.
Ultraviolet: we know that the Sun is a source of ultraviolet (or UV)
radiation, because it is the UV rays that cause our skin to burn! Stars
and other "hot" objects in space emit UV radiation.
 X-rays: your doctor uses them to look at your bones and your
dentist to look at your teeth. Hot gases in the Universe also emit
X-rays .
Gamma-rays:
radioactive materials (some natural and others made by man in things
like nuclear power plants) can emit gamma-rays. Big particle
accelerators that scientists use to help them understand what matter is
made of can sometimes generate gamma-rays. But the biggest gamma-ray
generator of all is the Universe! It makes gamma radiation in all kinds
of ways.
A Radio Wave is not a Gamma-Ray, a Microwave is not an X-ray ... or
is it?
Radio waves, visible light, X-rays, and all the other parts of the
electromagnetic spectrum are fundamentally the same thing,
electromagnetic radiation.
We may think that radio waves are completely different physical objects
or events than gamma-rays. They are produced in very different ways, and
we detect them in different ways. But are they really different things?
The answer is 'no'. Radio waves, visible light, X-rays, and all the
other parts of the electromagnetic spectrum are fundamentally the same
thing. They are all electromagnetic radiation.
Electromagnetic radiation can be described in terms of a stream of
photons, which are massless particles each traveling in a wave-like
pattern and moving at the speed of light. Each photon contains a certain
amount (or bundle) of energy, and all electromagnetic radiation consists
of these photons. The only difference between the various types of
electromagnetic radiation is the amount of energy found in the photons.
Radio waves have photons with low energies, microwaves have a little
more energy than radio waves, infrared has still more, then visible,
ultraviolet, X-rays, and ... the most energetic of all ... gamma-rays.
The electromagnetic spectrum can be expressed in terms of energy,
wavelength, or frequency.
Actually, the electromagnetic spectrum can be expressed in terms of
energy, wavelength, or frequency. Each way of thinking about the EM
spectrum is related to the others in a precise mathematical way. So why
do we have three ways of describing things, each with a different set of
physical units? After all, frequency is measured in cycles per second
(which is called a Hertz), wavelength is measured in meters, and energy
is measured in electron volts.
The answer is that scientists don't like to use big numbers when they
don't have to. It is much easier to say or write "two kilometers or 2
km" than "two thousand meters or 2,000 m". So generally, scientists use
whatever units are easiest for whatever they are working with. In radio
astronomy, astronomers tend to use wavelengths or frequencies. This is
because most of the radio part of the EM spectrum falls in the range
from a about 1 cm to 1 km, and 1 kilohertz (kHz) to 1 gigahertz (GHz).
The radio is a very broad part of the EM spectrum. Infrared astronomers
also use wavelength to describe their part of the EM spectrum. They tend
to use microns (or millionths of meters) for wavelengths, so that they
can say their part of the EM spectrum falls in the range 1 to 100
microns. Optical astronomers use wavelengths as well. In the older "CGS"
version of the metric system, the units used were angstroms. An Angstrom
is equal to 0.0000000001 meters (10-10 m in scientific notation)! In the
newer "SI" version of the metric system, we think of visible light in
units of nanometers or 0.000000001 meters (10-9 m). In this system, the
violet, blue, green, yellow, orange, and red light we know so well has
wavelengths between 400 and 700 nanometers. This range is only a small
part of the entire EM spectrum, so you can tell that the light we see is
just a little fraction of all the EM radiation around us! By the time
you get to the ultraviolet, X-ray, and gamma-ray regions of the EM
spectrum, lengths have become too tiny to think about any more. So
scientists usually refer to these photons by their energies, which are
measured in electron volts. Ultraviolet radiation falls in the range
from a few electron volts (eV) to a about 100 eV. X-ray photons have
energies in the range 100 eV to 100,000 eV (or 100 keV). Gamma-rays then
are all the photons with energies greater than 100 keV.
Why Do We Have to Go to Space to See All of the Electromagnetic
Spectrum?
Electromagnetic radiation from space is unable to reach the surface of
the Earth except at a very few wavelengths, such as the visible spectrum
and radio frequencies. Astronomers can get above enough of the Earth's
atmosphere to observe at some infrared wavelengths from mountain tops or
by flying their telescopes in an aircraft. Experiments can also be taken
up to altitudes as high as 35 km by balloons which can operate for
months. Rocket flights can take instruments all the way above the
Earth's atmosphere for just a few minutes before they fall back to
Earth, but a great many important first results in astronomy and
astrophysics came from just those few minutes of observations. For
long-term observations, however, it is best to have your detector on an
orbiting satellite ... and get above it all!
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