Radio is the wireless transmission of signals, by modulation of electromagnetic waves with frequencies below those of light.
Radio waves
Radio waves are a form of electromagnetic radiation, created whenever a
charged object (in normal radio transmission, 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 spectrum and diagram of radio transmission of an audio signal.
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. The word 'radio' is used to describe this phenomenon, and television, radio, radar, and cell phone transmissions are all classed as radio frequency emissions.
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