Electromagnetic radiation can be characterized as waves of electrical and magnetic energy radiating into space from a transmitting source with a speed of light. These waves are created by the acceleration of electrical charges. For example, the movement of charge in a transmitting radio antenna, i.e. the alternating current, generates electromagnetic waves that radiate away from the antenna and can be intercepted by a receiving antenna.

Radiation sources are characterized in two groups according to whether they cause ionization in the tissue or not.

Ionizing Radiation(f > 1016 Hz):

Ionizing Radiation includes alpha rays, beta rays, gamma rays, X-rays, neutrons, high-speed electrons, high-speed protons, and other atomic particles; but such term does not include sound or radio waves, or visible light, or infrared or ultraviolet light. When this type of radiation is absorbed by the tissue it causes ionization.

Nonionizing Radiation (EM Radiation):

Nonionizing Radiation is a general term for both radiations and fields that form part of the electromagnetic spectrum having insufficient radiated energy to produce ionization in the medium through which it passes. Nonionizing radiation includes electric and magnetic fields, electromagnetic fields and radiation in the frequency range 0-300 GHz, optical radiations (ultraviolet, visible light, and infrared) and ultrasound (frequencies above 16 kHz).

Electromagnetic Spectrum: