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 energy.

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

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