Process by which energy is emitted from a source and propagated through the surrounding medium, or the energy involved in this process.
Radiation consists of a flow of atomic or subatomic particles or of waves. Familiar examples are light (a form of electromagnetic radiation ) and sound (a form of acoustic radiation). Both electromagnetic and acoustic radiation can be described as waves with a range of frequencies and intensities. Electromagnetic radiation is also often treated as discrete packets of energy, called photons . All matter is constantly bombarded by radiation from cosmic and terrestrial sources, and radioactive elements emit several types of radiation (see radioactivity ). See also Cherenkov ratiation , Hawking radiation , infrared radiation , synchrotron radiation , thermal radiation , ultraviolet radiation .