The term "radiation" simply refers to the emission and propagation of energy in the form of waves or particles.
Electromagnetic (EM) radiation is a radiation consisting of particles called photons that act in some respects like a wave. It is caused by an oscillating electric charge.
EM is characterised by the frequency of the wave. The frequency is the number of full waves that travel past a point in a given time - this is the number of crests or equivalently the number of troughs. When the unit of time is a second the frequency is expressed in Hertz (abbreviation: Hz). Therefore a frequency of 434 MHz indicates a wave in which 434 million waves travel past a point in each second.
All EM waves travel at the same speed. This is a universal constant known as the velocity of light (c) and is approximately 300,000 km / sec (3 x 108 m / s) – the phase velocity.
The frequency and wavelength of a wave are inversely proportional to one another, to calculate the speed of the wave; one is divided by the other. For example, in the case of 434 MHz this implies an in vacuo wavelength of 61.9 cm (less inside matter such as a patient's body).
It includes the range of the EM spectrum: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, x-rays and gamma rays. The divisions between these parts of the spectrum reflect differences in how the energy is created or detected rather than specific phenomena. For example, gamma rays (those produced by a LINAC) are basically high energy x-rays.
EM radiation carries energy – each photon is an indivisible packet of energy (E) and the amount of this energy is proportional to its frequency). Photons have no mass but they still carry momentum (p), that is to say, they push things that they strike, albeit a very small push.
EM radiation is classified into ionising and non-ionising radiation, based on whether it is capable of ionising atoms and breaking chemical bonds. Radiation with energies in the ultraviolet (UV) band or higher is ionising.
Radiowaves are not ionising – the only known biological effect they have on the human body is heating.