Radio is a technology that allows for the transmission of signals by modulation of electromagnetic waves. These waves travel (propagate) through the air and the vacuum of space equally well, not requiring a medium of transport.
A radio wave is created whenever a charged object accelerates with a frequency that lies in the radio frequency (RF) portion of the electromagnetic spectrum. By contrast, other types of emissions which fall outside the RF range are gamma rays, X-rays, infrared & ultraviolet light, and light visible to humans.
When a radio wave passes a wire, it induces a moving electric charge (voltage) that 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 in the class of radio frequency emissions.
The theoretical basis of the propagation of electromagnetic waves was first described in 1873 by James Clerk Maxwell in his paper to the Royal Society A dynamical theory of the electromagnetic field, which followed his work between 1861 and 1865.
It was Heinrich Rudolf Hertz who, between 1886 and 1888, first validated Maxwell’s theory through experiment, demonstrating that radio radiation had all the properties of waves (then called Hertzian waves), and discovering that the electromagnetic equations could be reformulated into a partial differential equation called the wave equation.