<P> Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light . Radio waves have frequencies as high as 300 gigahertz (GHz) to as low as 30 hertz (Hz). At 300 GHz, the corresponding wavelength is 1 mm, and at 30 Hz is 10,000 km . Like all other electromagnetic waves, radio waves travel at the speed of light . They are generated by electric charges undergoing acceleration, such as time varying electric currents . Naturally occurring radio waves are emitted by lightning and astronomical objects . </P> <P> Radio waves are generated artificially by transmitters and received by radio receivers, using antennas . Radio waves are very widely used in modern technology for fixed and mobile radio communication, broadcasting, radar and other navigation systems, communications satellites, wireless computer networks and many other applications . Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere; long waves can diffract around obstacles like mountains and follow the contour of the earth (ground waves), shorter waves can reflect off the ionosphere and return to earth beyond the horizon (skywaves), while much shorter wavelengths bend or diffract very little and travel on a line of sight, so their propagation distances are limited to the visual horizon . </P>

Are radio waves electromagnetic waves or sound waves