Contrary to popular belief, outer space is not
empty. It is filled with electromagnetic
radiation that crisscrosses the universe.
This radiation comprises the spectrum of
energy ranging from radio waves on one
end to gamma rays on the other. It is called
the electromagnetic spectrum because this
radiation is associated with electric and
magnetic fields that transfer energy as they
travel through space. Because humans can
see it, the most familiar part of the
electromagnetic spectrum is visible lightQ
red, orange, yellow, green, blue, and violet.
Like expanding ripples in a pond after a
pebble has been tossed in, electromagnetic
radiation travels across space in the form of
waves. These waves travel at the speed of
light--300,000 kilometers per second.
Their wavelengths, the distance from wave
crest to wave crest, vary from thousands of
kilometers across, in the case of the longest
radio waves, to smaller than the diameter of
an atom, in the cases of the smallest x-rays
and gamma rays.
Electromagnetic radiation has properties of both waves and particles. What we detect depends on the method we use to study it. The beautiful colors that appear in a soap film or in the dispersion of light from a diamond are best described as waves. The light that strikes a solar cell to produce an
electric current is best described as a
particle. When described as particles,
individual packets of electromagnetic energy
are called photons. The amount of energy a
photon of light contains depends upon its
wavelength. Electromagnetic radiation with
long wavelengths contains little energy.
Electromagnetic radiation with short
wavelengths contains a great amount of
Scientists name the different regions of the electromagnetic spectrum according to their wavelengths. (See figure 1.) Radio waves have the longest wavelengths, ranging from a few centimeters from crest to crest to thousands of kilometers. Microwaves range from a few centimeters to about 0.1 cm. Infrared radiation falls between 700 nanometers and 0.1 cm. (Nano means one billionth. Thus 700 nanometers is a distance equal to 700 billionths or 7 x 10 -7 meter.) Visible light is a very narrow band of radiation ranging from 400 to 700 nanometers. For comparison, the thickness of a sheet of household plastic wrap could contain about 50 visible light waves arranged end to end. Below visible light is the slightly broader band of ultraviolet light that lies between 10 and 300 nanometers. X-rays follow ultraviolet light and diminish into the hundred-billionth of a meter range. Gamma rays fall in the trillionth of a meter range.
The wavelengths of x-rays and gamma rays