Editing Electromagnetic radiation (section)

== Electromagnetic spectrum ==
{{Main|Electromagnetic spectrum}}
[[File:EM_spectrum_updated.svg|thumb|upright=2.25|right|[[Electromagnetic spectrum]] with visible light highlighted. The bottom graph (visible spectrum) shows wavelength in units of nanometers (nm).]]
[[File:Light spectrum.svg|right|frame|'''Legend:'''<br />
γ = [[Gamma ray]]s<br />
<br />
HX = Hard [[X-ray]]s<br />
SX = Soft X-rays<br />
<br />
EUV = Extreme-[[ultraviolet]]<br />
NUV = Near-ultraviolet<br />
<br />
[[Visible light]] (colored bands)<br />
<br />
NIR = Near-[[infrared]]<br />
MIR = Mid-infrared<br />
FIR = Far-infrared<br />
<br />
EHF = [[Extremely high frequency]] (microwaves)<br />
SHF = [[Super-high frequency]] (microwaves)<br />
<br />
UHF = [[Ultrahigh frequency]] (radio waves)<br />
VHF = [[Very high frequency]] (radio)<br />
HF = [[High frequency]] (radio)<br />
MF = [[Medium frequency]] (radio)<br />
LF = [[Low frequency]] (radio)<br />
VLF = [[Very low frequency]] (radio)<br />
VF = [[Voice frequency]]<br />
ULF = [[Ultra-low frequency]] (radio)<br />
SLF = [[Super-low frequency]] (radio)<br />
ELF = [[Extremely low frequency]] (radio)]]

EM radiation (the designation 'radiation' excludes static electric and magnetic and [[near and far field|near fields]]) is classified by wavelength into [[radio wave|radio]], [[microwave]], [[infrared]], [[visible spectrum|visible]], [[ultraviolet]], [[X-ray]]s, and [[gamma rays]]. Arbitrary electromagnetic waves can be expressed by [[Fourier analysis]] in terms of [[sinusoidal]] waves ([[monochromatic radiation]]), which in turn can each be classified into these regions of the EMR spectrum.

For certain classes of EM waves, the waveform is most usefully treated as ''random'', and then spectral analysis must be done by slightly different mathematical techniques appropriate to random or [[stochastic process]]es. In such cases, the individual frequency components are represented in terms of their ''power'' content, and the phase information is not preserved. Such a representation is called the [[power spectral density]] of the random process. Random electromagnetic radiation requiring this kind of analysis is, for example, encountered in the interior of stars, and in certain other very wideband forms of radiation such as the [[Zero point field|zero-point wave field]] of the electromagnetic vacuum.

The behavior of EM radiation and its interaction with matter depends on its frequency, and changes qualitatively as the frequency changes. Lower frequencies have longer wavelengths, and higher frequencies have shorter wavelengths, and are associated with photons of higher energy. There is no fundamental limit known to these wavelengths or energies, at either end of the spectrum, although photons with energies near the [[Planck energy]] or exceeding it (far too high to have ever been observed) will require new physical theories to describe.

=== Radio and microwave ===
{{Main|Radio wave|Microwave}}
Electromagnetic radiation phenomena with wavelengths ranging from one meter to one millimeter are called microwaves; with frequencies between 300&nbsp;MHz (0.3&nbsp;GHz) and 300&nbsp;GHz. When radio waves impinge upon a [[Electrical conductor|conductor]], they couple to the conductor, travel along it, and [[radio frequency induction|induce]] an electric current on the conductor surface by moving the electrons of the conducting material in correlated bunches of charge. At radio and microwave frequencies, EMR interacts with matter largely as a bulk collection of charges which are spread out over large numbers of affected atoms. In [[electrical conductor]]s, such induced bulk movement of charges ([[electric current]]s) results in absorption of the EMR, or else separations of charges that cause generation of new EMR (effective reflection of the EMR). An example is absorption or emission of radio waves by antennas, or absorption of microwaves by water or other molecules with an [[electric dipole moment]], as for example inside a [[microwave oven]]. These interactions produce either electric currents or heat, or both.

=== Infrared ===
{{Main|Infrared}}
Like radio and microwave, infrared (IR) is reflected by metals (and also most EMR, well into the ultraviolet range). However, unlike lower-frequency radio and microwave radiation, infrared EMR commonly interacts with dipoles present in single molecules, which change as atoms vibrate at the ends of a single chemical bond. It is consequently absorbed by a wide range of substances, causing them to increase in temperature as the vibrations dissipate as heat. The same process, run in reverse, causes bulk substances to radiate in the infrared spontaneously (see [[thermal radiation]] section below).

Infrared radiation is divided into spectral subregions. While different subdivision schemes exist,<ref>{{cite web|last=Henderson |first=Roy |url=http://info.tuwien.ac.at/iflt/safety/section1/1_1_1.htm |title=Wavelength considerations |publisher=Instituts für Umform- und Hochleistungs |access-date=18 October 2007 |archive-url = https://web.archive.org/web/20071028072110/http://info.tuwien.ac.at/iflt/safety/section1/1_1_1.htm |archive-date = 28 October 2007}}</ref><ref>{{cite web |url=http://www.ipac.caltech.edu/Outreach/Edu/Regions/irregions.html |title=Near, Mid and Far-Infrared |publisher=NASA IPAC |access-date=4 April 2007 |url-status=dead |archive-url=https://archive.today/20120529/http://www.ipac.caltech.edu/Outreach/Edu/Regions/irregions.html |archive-date=29 May 2012 }}</ref> the spectrum is commonly divided as near-infrared (0.75–1.4&nbsp;μm), short-wavelength infrared (1.4–3&nbsp;μm), mid-wavelength infrared (3–8&nbsp;μm), long-wavelength infrared (8–15&nbsp;μm) and [[far infrared]] (15–1000&nbsp;μm).<ref name="Byrnes">{{Cite book|last=Byrnes |first=James |title=Unexploded Ordnance Detection and Mitigation |url=https://archive.org/details/unexplodedordnan00abry |url-access=limited |publisher=Springer |year=2009 |pages=[https://archive.org/details/unexplodedordnan00abry/page/n29 21]–22 |isbn=978-1-4020-9252-7|bibcode=2009uodm.book.....B }}</ref>

Some animals, such as [[Infrared sensing in snakes|for snakes]], have thermo-sensitive membranes (pit organs) that can detect temperature differences, allowing them to sense infrared radiation.<ref>{{Cite journal |last=Gracheva |first=Elena O. |last2=Ingolia |first2=Nicholas T. |last3=Kelly |first3=Yvonne M. |last4=Cordero-Morales |first4=Julio F. |last5=Hollopeter |first5=Gunther |last6=Chesler |first6=Alexander T. |last7=Sánchez |first7=Elda E. |last8=Perez |first8=John C. |last9=Weissman |first9=Jonathan S. |last10=Julius |first10=David |date=2010 |title=Molecular basis of infrared detection by snakes |url=https://www.nature.com/articles/nature08943 |journal=Nature |language=en |volume=464 |issue=7291 |pages=1006–1011 |doi=10.1038/nature08943 |issn=0028-0836 |pmc=2855400 |pmid=20228791}}</ref>

=== Visible light ===
{{Main|Light}}
Natural sources produce EM radiation across the spectrum. EM radiation with a [[wavelength]] between approximately 400 [[nanometre|nm]] and 700&nbsp;nm is directly detected by the [[human eye]] and perceived as visible light. Other wavelengths, especially nearby infrared (longer than 700&nbsp;nm) and ultraviolet (shorter than 400&nbsp;nm) are also sometimes referred to as light.

As frequency increases into the visible range, photons have enough energy to change the bond structure of some individual molecules. It is not a coincidence that this happens in the visible range, as the [[Visual system|mechanism of vision]] involves the change in bonding of a single molecule, [[retinal]], which absorbs a single photon. The change in retinal causes a change in the shape of the [[rhodopsin]] protein it is contained in, which starts the biochemical process that causes the [[retina]] of the human eye to sense the light.

Visible light is able to affect only a tiny percentage of all molecules. Usually not in a permanent or damaging way, rather the photon excites an electron which then emits another photon when returning to its original position. This is the source of color produced by most dyes. Retinal is an exception. When a photon is absorbed, the [[Retinal#Visual cycle|retinal permanently changes structure from cis to trans]], and requires a protein to convert it back, i.e. reset it to be able to function as a light detector again.

[[Photosynthesis]] becomes possible in this range as well, for the same reason. A single molecule of [[chlorophyll]] is excited by a single photon. In plant tissues that conduct photosynthesis, [[carotenoids]] act to quench electronically excited chlorophyll produced by visible light in a process called [[non-photochemical quenching]], to prevent reactions that would otherwise interfere with photosynthesis at high light levels.

Limited evidence indicate that some [[reactive oxygen species]] are created by visible light in skin, and that these may have some role in [[photoaging]], in the same manner as [[ultraviolet A]].<ref name="Liebel-2012">{{Cite journal |last1=Liebel |first1=F. |last2=Kaur |first2=S. |last3=Ruvolo |first3=E. |last4=Kollias |first4=N. |last5=Southall |first5=M. D. |year=2012 |title=Irradiation of Skin with Visible Light Induces Reactive Oxygen Species and Matrix-Degrading Enzymes |journal=Journal of Investigative Dermatology |volume=132 |issue=7 |pages=1901–1907 |doi=10.1038/jid.2011.476 |pmid=22318388 |doi-access=free}}</ref>

Infrared, microwaves, and radio waves are known to damage molecules and biological tissue only by bulk heating, not excitation from single photons of the radiation.

=== Ultraviolet ===
{{Main|Ultraviolet}}
As frequency increases into the ultraviolet, photons now carry enough energy (about three [[electron volt]]s or more) to excite certain doubly bonded molecules into permanent chemical rearrangement. In [[DNA]], this causes lasting damage. DNA is also indirectly damaged by reactive oxygen species produced by ultraviolet A (UVA), which has energy too low to damage DNA directly. This is why ultraviolet at all wavelengths can damage DNA, and is capable of causing cancer, and (for [[UVB]]) skin burns (sunburn) that are far worse than would be produced by simple heating (temperature increase) effects.

At the higher end of the ultraviolet range, the energy of photons becomes large enough to impart enough energy to electrons to cause them to be liberated from the atom, in a process called [[photoionisation]]. The energy required for this is always larger than about 10 [[electron volt]] (eV) corresponding with wavelengths smaller than 124&nbsp;nm (some sources suggest a more realistic cutoff of 33&nbsp;eV, which is the energy required to ionize water). This high end of the ultraviolet spectrum with energies in the approximate ionization range, is sometimes called "extreme UV". Ionizing UV is strongly filtered by the Earth's atmosphere.<ref>{{Citation |last= |first= |title=Solar and Ultraviolet Radiation |date=2012 |work=Radiation |url=https://www.ncbi.nlm.nih.gov/books/NBK304366/ |access-date=2025-03-24 |publisher=International Agency for Research on Cancer |language=en}}</ref>

=== X-rays and gamma rays ===
{{Main|X-rays|Gamma rays}}
Electromagnetic radiation composed of photons that carry minimum-ionization energy, or more (which includes the entire spectrum with shorter wavelengths), is therefore termed [[ionizing radiation]]. (Many other kinds of ionizing radiation are made of non-EM particles.) Electromagnetic-type ionizing radiation extends from the extreme ultraviolet to all higher frequencies and shorter wavelengths, which means that all X-rays and gamma rays qualify. These are capable of the most severe types of molecular damage, which can happen in biology to any type of biomolecule, including mutation and cancer,<ref name="Baeyens-2023">{{Citation |last1=Baeyens |first1=Ans |title=Basic Concepts of Radiation Biology |date=2023 |work=Radiobiology Textbook |pages=25–81 |editor-last=Baatout |editor-first=Sarah |url=https://link.springer.com/chapter/10.1007/978-3-031-18810-7_2 |access-date=2025-03-24 |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-031-18810-7_2 |isbn=978-3-031-18810-7 |last2=Abrantes |first2=Ana Margarida |last3=Ahire |first3=Vidhula |last4=Ainsbury |first4=Elizabeth A. |last5=Baatout |first5=Sarah |last6=Baselet |first6=Bjorn |last7=Botelho |first7=Maria Filomena |last8=Boterberg |first8=Tom |last9=Chevalier |first9=Francois}}</ref> and often at great depths below the skin, since the higher end of the X-ray spectrum, and all of the gamma ray spectrum, penetrate matter.