Radio waves are a type of
electromagnetic radiation with
wavelengths in the
electromagnetic spectrum longer than
infrared light. Radio waves have
frequencies from 300
GHz to as low as 3
kHz, and corresponding wavelengths ranging from to . Like all other electromagnetic waves, they travel at the speed of light. Naturally occurring radio waves are made by
lightning, or by
astronomical objects. Artificially generated radio waves are used for fixed and mobile
radio communication,
broadcasting,
radar and other navigation systems,
communications satellites, computer networks and innumerable other applications. Radio waves are generated by
radio transmitters and received by
radio receivers. 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.