An electromagnetic wave possessing a long wavelength (ranging from about 10^-3 to 10⁴m and a low frequency (from about 10⁵ to 10¹¹Hz that travels at the speed of light. Included in the radio wave part of the spectrum are: microwaves, used for both communications and for cooking; ultra high- and very high-frequency waves, used for television and FM (frequency modulation) radio communications; and short, medium, and long waves, used for AM (amplitude modulation) radio communications. Radio waves that are used for communications have all been modulated to carry information. Certain astronomical objects emit radio waves, which may be detected and studied using radio telescopes.

There is a layer in the atmosphere, called the ionosphere, where gas molecules are separated from their electrons by radiation from the Sun. When radio waves reach the ionosphere, they produce movements or oscillations of the electrons. As the electrons oscillate they produce electromagnetic waves that are identical to the radio waves with which the electrons were stimulated. The radio waves are reflected back and can be detected by a receiver. It is important to note that the reflected radio waves come from the oscillating electrons in the ionosphere.