radio wave

Electromagnetic wave possessing a relatively long wavelength (ranging from about 10^-3 m upwards) and a relatively low frequency (from about 3 × 10¹¹ Hz downwards) and travelling 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 are modulated (see modulation) 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 of certain frequencies 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 net effect is that radio waves originating from the ground are reflected back and can be detected by a receiver. Radio waves of the same frequencies reaching the ionosphere from space are also reflected and do not reach the Earth's surface.