Editing Radiation (section)

{{Short description|Waves or particles moving through space}}
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[[File:Alfa beta gamma radiation penetration.svg|upright=1.5|thumb|right|Illustration of the relative abilities of three different types of [[ionizing radiation]] to penetrate solid matter. Typical alpha particles (α) are stopped by a sheet of paper, while beta particles (β) are stopped by 3mm aluminum foil. Gamma radiation (γ) is dampened when it penetrates lead. Note caveats in the text about this simplified diagram.{{clarify|date=March 2020|reason=The text does not mention the diagram. The caveats should probably be listed in a note using Template:Efn}}]]

[[File:Radioactive.svg|thumb|upright|The international symbol for types and levels of ionizing radiation (radioactivity) that are unsafe for [[Radiation shield|unshielded]] humans. Radiation, in general, exists throughout nature, such as in light and sound.]]

In [[physics]], '''radiation''' is the emission or transmission of [[energy]] in the form of [[wave]]s or [[particle]]s through space or a material medium.<ref>{{cite web |url=http://scienceworld.wolfram.com/physics/Radiation.html |title=Radiation |last=Weisstein |first=Eric W. |website=Eric Weisstein's World of Physics |publisher=Wolfram Research |access-date=2014-01-11}}</ref><ref>{{cite web |url=http://www.thefreedictionary.com/radiation |title=Radiation |website=The free dictionary by Farlex |publisher=Farlex, Inc. |access-date=2014-01-11}}</ref> This includes:
* ''[[electromagnetic radiation]]'' consisting of [[photon]]s, such as [[radio wave]]s, [[microwaves]], [[infrared]], [[visible light]], [[ultraviolet]], [[x-ray]]s, and [[Gamma ray|gamma radiation (γ)]]
* ''[[particle radiation]]'' consisting of particles of non-zero [[rest energy]], such as [[alpha radiation]] (α), [[beta radiation]] (β), proton radiation and [[neutron radiation]]
* ''[[acoustics|acoustic]] radiation'', such as [[ultrasound]], [[sound]], and [[seismic wave]]s, all dependent on a physical [[transmission medium]]
* ''[[gravitational radiation]]'', in the form of gravitational waves, ripples in [[spacetime]]

Radiation is often categorized as either ''[[ionizing radiation|ionizing]]'' or ''[[non-ionizing radiation|non-ionizing]]'' depending on the energy of the radiated particles. Ionizing radiation carries more than 10 [[electron volt|electron volts (eV)]], which is enough to [[ionize]] atoms and molecules and break [[chemical bond]]s. This is an important distinction due to the large difference in harmfulness to living organisms. A common source of ionizing radiation is [[Radioactive decay|radioactive materials]] that emit α, β, or [[gamma radiation|γ radiation]], consisting of [[helium nuclei]], [[electron]]s or [[positron]]s, and [[photon]]s, respectively. Other sources include [[X-ray]]s from medical [[radiography]] examinations and [[muon]]s, [[meson]]s, positrons, [[neutron]]s and other particles that constitute the secondary [[cosmic ray]]s that are produced after primary cosmic rays interact with [[Earth's atmosphere]].

Gamma rays, X-rays, and the higher energy range of ultraviolet light constitute the ionizing part of the [[electromagnetic spectrum]]. The word "ionize" refers to the breaking of one or more electrons away from an atom, an action that requires the relatively high energies that these electromagnetic waves supply. Further down the spectrum, the non-ionizing lower energies of the lower ultraviolet spectrum cannot ionize atoms, but can disrupt the inter-atomic bonds that form molecules, thereby breaking down molecules rather than atoms; a good example of this is sunburn caused by long-[[wavelength]] solar ultraviolet. The waves of longer wavelength than UV in visible light, infrared, and microwave frequencies cannot break bonds but can cause vibrations in the bonds which are sensed as [[heat]]. Radio wavelengths and below generally are not regarded as harmful to biological systems. These are not sharp delineations of the energies; there is some overlap in the effects of specific [[frequency|frequencies]].<ref>{{cite web| url=https://www.cdc.gov/nceh/radiation/nonionizing_radiation.html| title=The Electromagnetic Spectrum| publisher=Centers for Disease Control and Prevention| date=7 December 2015| access-date=29 August 2018}}</ref>

The word "radiation" arises from the phenomenon of waves ''radiating'' (i.e., traveling outward in all directions) from a source. This aspect leads to a system of [[radiometry|measurements and physical units]] that apply to all types of radiation. Because such radiation expands as it passes through space, and as its energy is conserved (in vacuum), the intensity of all types of radiation from a [[point source]] follows an [[inverse-square law]] in relation to the distance from its source. Like any ideal law, the inverse-square law approximates a measured radiation intensity to the extent that the source approximates a geometric point.