Radio is the wireless transmission of signals, by modulation of electromagnetic waves with frequencies below those of light. more...
Radio waves are a form of electromagnetic radiation, created whenever a charged object (e.g. an electron) accelerates with a frequency that lies in the radio frequency (RF) portion of the electromagnetic spectrum. In radio, this acceleration is caused by an alternating current in an antenna. Radio frequencies occupy the range from a few tens of hertz to a few hundred gigahertz.
Other types of electromagnetic radiation, with frequencies above the RF range are infrared, visible light, ultraviolet, X-rays and gamma rays. Since the energy of an individual photon of radio frequency is too low to remove an electron from an atom, radio waves are classified as non-ionizing radiation.
Electromagnetic radiation travels (propagates) by means of oscillating electromagnetic fields that pass through the air and the vacuum of space equally well, and does not require a medium of transport (such as the aether). When radio waves pass an electrical conductor, the oscillating electric or magnetic field (depending on the shape of the conductor) induces an alternating current and voltage in the conductor. This can be transformed into audio or other signals that carry information. Although the word 'radio' is used to describe this phenomenon, the transmissions which we know as television, radio, radar, and cell phone are all classed as radio frequency emissions.
History and invention
Origin of the word
Originally, radio technology was called 'wireless telegraphy', which was shortened to 'wireless'. The prefix radio- in the sense of wireless transmission is first recorded in the word radioconductor, coined by the French physicist Edouard Branly in 1897 and based on the verb to radiate. 'Radio' as a noun is said to have been coined by advertising expert Waldo Warren (White 1944). The word appears in a 1907 article by Lee de Forest, was adopted by the United States Navy in 1912 and became common by the time of the first commercial broadcasts in the United States in the 1920s. (The noun 'broadcasting' itself came from an agricultural term, meaning 'scattering seeds'.) The American term was then adopted by other languages in Europe and Asia, although Britain retained the term 'wireless' until the mid-20th century.
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