Radio

          Radio is a form of communication in which intelligence is transmitted without wires from one point to another by means of electromagnetic waves. Early forms of communication over great distances were the telephone and the telegraph. They require wires between the sender and the receiver.

          Radio, on the other hand, requires on such physical connection. It relies on the radiation of energy from a transmitting antenna in the form of radio waves. This radio waves, traveling at the speed of light (300.000 km/sec.) carry the information. When the waves arrive at a receiving antenna, the original information contained in the radio waves is retrieved and presented in an understandable form, such as sound from a loudspeaker.

          The principles of radio had been demonstrated in the early 1800s by scientists, such as Michael Faraday and Joseph Henry. They had individual induced (produced) a current in another wire that was not physically connected to the first.

          All radio programs or signals are transmitted within a broad spectrum of waves called the electromagnetic spectrum. This spectrum is responsible for producing a variety of waves, including those that can be detected by  man-made machines, such as X-rays. Radio waves are among the many types of electromagnetic waves that travel within the electromagnetic spectrum. Radio waves can be defined by their frequency (in hertz, after Heinrich Hertz, who first produced radio waves electronically), which is the number of times they pass through a complete cycle per second. Besides, we can define radio waves by their wavelength, which is determined by the distance (in kilometers) traveled from the crest of one wave to the crest of the next.

  Source: Encyclopedia of Knowledge