 Radio waves, radio waves are a type of electromagnetic radiation with wave lengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as high as 300 J HZ2 as low as 30 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-parrying electric currents. Naturally a current radio waves are emitted by lightning and astronomical objects. 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 with sky waves 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. To prevent interference between different users, the artificial generation and use of radio waves is strictly regulated by law, coordinated by an international body called the International Telecommunications Union ITU which defines radio waves as electromagnetic waves of frequencies arbitrarily lower than 300 GHz, propagated in space without artificial guide. The radio spectrum is divided into a number of radio bands on the basis of frequency, allocated to different uses.