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[[Image:EM Spectrum Properties edit.svg|thumb|330px|A diagram of the electromagnetic spectrum, showing various properties across the range of frequencies and wavelengths.]]
The '''electromagnetic spectrum''' is the [[Spectrum|range]] of all possible frequencies of [[electromagnetic radiation]].<ref>{{cite web|url=http://imagine.gsfc.nasa.gov/docs/dict_ei.html#em_spectrum|title=Imagine the Universe! Dictionary}}</ref> The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object.

The electromagnetic spectrum extends from low frequencies used for modern [[radio]] communication to [[gamma radiation]] at the short-[[wavelength]] (high-frequency) end, thereby covering wavelengths from thousands of [[kilometre]]s down to a [[fraction]] of the size of an [[atom]]. It is for this reason that the electromagnetic spectrum is highly studied for spectroscopic purposes to characterize matter.<ref name="em-spectrum">{{cite web|last=Mehta |first=Akul |url=http://pharmaxchange.info/press/2011/08/introduction-to-the-electromagnetic-spectrum-and-spectroscopy/ |title=Introduction to the Electromagnetic Spectrum and Spectroscopy |publisher=Pharmaxchange.info |date= |accessdate=2011-11-08}}</ref> The limit for long wavelength is the size of the [[universe]] itself, while it is thought that the short wavelength limit is in the vicinity of the [[Planck length]],<ref>{{cite book |title=Basic Electronics Engineering |author=U. A. Bakshi, A. P. Godse |url=http://books.google.com/books?id=n0RMHUQUUY4C&dq=Basic+Electronics+Engineering&source=gbs_navlinks_s |pages=8 - 10 |year=2009 |isbn=9788184315806 |publisher=Technical Publications |accessdate=2011-10-16}}</ref> although in principle the spectrum is [[infinity|infinite]] and [[Continuum (theory)|continuous]].

[[Image:Light spectrum.svg|right|frame|Legend<ref>[http://cbst.ucdavis.edu/education/courses/winter-2006-IST8A/ist8a_2006_01_09light.pdf What is Light?] – [[UC Davis]] lecture slides</ref><ref>{{cite web|author=Glenn Elert |url=http://hypertextbook.com/physics/electricity/em-spectrum/ |title=The Electromagnetic Spectrum, The Physics Hypertextbook |publisher=Hypertextbook.com |accessdate=2010-10-16}}</ref><ref>{{cite web|url=http://www.vlf.it/frequency/bands.html |title=Definition of frequency bands on |publisher=Vlf.it |accessdate=2010-10-16}}</ref>
{| border="0"
|-
|γ= [[Gamma ray]]s|||MIR= Mid infrared|||HF= [[High frequency|High freq.]]
|-
|HX= Hard [[X-ray]]s ||FIR= Far infrared||MF= [[Medium frequency|Medium freq.]]
|-
|SX= Soft X-rays||[[Radio waves]]||LF= [[Low frequency|Low freq.]]
|-
|EUV= Extreme [[ultraviolet]]||EHF= [[Extremely high frequency|Extremely high freq.]]||VLF= [[Very low frequency|Very low freq.]]
|-
|NUV= [[Near ultraviolet]]||SHF= [[Super high frequency|Super high freq.]]||VF/ULF= [[Voice frequency|Voice freq.]]
|-
|[[Visible light]]||UHF= [[Ultra high frequency|Ultra high freq.]]||SLF= [[Super low frequency|Super low freq.]]
|-
|NIR= Near [[Infrared]]||VHF= [[Very high frequency|Very high freq.]]||ELF= [[Extremely low frequency|Extremely low freq.]]
|-
|
|
|Freq=[[Frequency]]
|}]]

==Range of the spectrum==
Electromagnetic waves are typically described by any of the following three physical properties: the [[frequency]] ''f'', [[wavelength]] [[lambda|λ]], or [[photon]] [[energy]] ''E''. Frequencies range from {{val|2.4|e=23|u=Hz}} (1 [[GeV]] gamma rays) down to the local [[plasma frequency]] of the ionized interstellar medium (~1&nbsp;kHz). Wavelength is inversely proportional to the wave frequency,<ref name="em-spectrum"/> so gamma rays have very short wavelengths that are fractions of the size of [[atom]]s, whereas wavelengths can be as long as the universe. Photon energy is directly proportional to the wave frequency, so gamma rays have the highest energy (around a billion [[electron volt]]s) and radio waves have very low energy (around a [[femto]] electron volts). These relations are illustrated by the following equations:
:<math>f = \frac{c}{\lambda}, \quad\text{or}\quad f = \frac{E}{h}, \quad\text{or}\quad E=\frac{hc}{\lambda},</math>
where:
*''c'' = {{val|299792458|u=m/s}} is the [[speed of light]] in vacuum and
*''h'' = {{val|6.62606896|(33)|e=-34|u=J s}} = {{val|4.13566733|(10)|e=-15|u=eV s}} is [[Planck's constant]].<ref name="CODATA">{{CODATA2006|url=http://physics.nist.gov/cgi-bin/cuu/Value?h}}</ref>

Whenever electromagnetic waves exist in a [[Transmission medium|medium]] with [[matter]], their wavelength is decreased. Wavelengths of electromagnetic radiation, no matter what medium they are traveling through, are usually quoted in terms of the ''vacuum wavelength'', although this is not always explicitly stated.

Generally, EM radiation is classified by wavelength into [[radio wave]], [[microwave]], [[infrared]], the [[visible region]] we perceive as light, [[ultraviolet]], [[X-ray]]s and [[gamma rays]]. The behavior of EM radiation depends on its wavelength. When EM radiation interacts with single atoms and molecules, its behaviour also depends on the amount of energy per quantum (photon) it carries.

[[Spectroscopy]] can detect a much wider region of the EM spectrum than the visible range of 400&nbsp;nm to 700&nbsp;nm. A common laboratory spectroscope can detect wavelengths from 2&nbsp;nm to 2500&nbsp;nm. Detailed information about the physical properties of objects, gases, or even stars can be obtained from this type of device. Spectroscopes are widely used in [[astrophysics]]. For example, many [[hydrogen]] [[atom]]s [[Emission (electromagnetic radiation)|emit]] a [[radio wave]] photon which has a wavelength of 21.12&nbsp;cm. Also, frequencies of 30 [[hertz|Hz]] and below can be produced by and are important in the study of certain stellar nebulae<ref>{{cite web|url=http://www.cv.nrao.edu/course/astr534/Pulsars.html|title=Essential Radio Astronomy: Pulsar Properties|author=J. J. Condon and S. M. Ransom |publisher=[[National Radio Astronomy Observatory]]|accessdate=2008-01-05}}</ref> and frequencies as high as {{val|2.9|e=27|u=Hz}} have been detected from astrophysical sources.<ref>{{cite journal|doi= 10.1086/513696|title=Discovery of TeV Gamma‐Ray Emission from the Cygnus Region of the Galaxy|journal=The Astrophysical Journal Letters|date=2007-03-20|author=A. A. Abdo ''et al.''|volume=658|pages=L33|bibcode=2007ApJ...658L..33A|arxiv = astro-ph/0611691 }}</ref>

==Rationale==
Electromagnetic radiation interacts with matter in different ways in different parts of the spectrum. The types of interaction can be so different that it seems to be justified to refer to different types of radiation. At the same time, there is a continuum containing all these "different kinds" of electromagnetic radiation. Thus we refer to a spectrum, but divide it up based on the different interactions with matter.

{| class="wikitable"
|-
! Region of the spectrum
! Main interactions with matter
|-
|[[Radio wave|Radio]]
|Collective oscillation of charge carriers in bulk material ([[plasma oscillation]]). An example would be the oscillation of the electrons in an [[antenna (radio)|antenna]].
|-
|[[Microwave]] through far [[infrared]]
|Plasma oscillation, molecular rotation
|-
|Near [[infrared]]
|Molecular vibration, plasma oscillation (in metals only)
|-
|[[Light|Visible]]
|Molecular electron excitation (including pigment molecules found in the human retina), plasma oscillations (in metals only)
|-
|[[Ultraviolet]]
|Excitation of molecular and atomic valence electrons, including ejection of the electrons ([[photoelectric effect]])
|-
|[[Xray|X-rays]]
|Excitation and ejection of core atomic electrons, [[Compton scattering]] (for low atomic numbers)
|-
|[[Gamma ray]]s
|Energetic ejection of core electrons in heavy elements, [[Compton scattering]] (for all atomic numbers), excitation of atomic nuclei, including dissociation of nuclei
|-
|High energy [[gamma ray]]s
|Creation of [[Virtual pair|particle-antiparticle pairs]]. At very high energies a single photon can create a shower of high energy particles and antiparticles upon interaction with matter.
|}

==Types of radiation==
[[Image:Electromagnetic-Spectrum.png|thumb|300px|The '''electromagnetic spectrum''']]
The types of electromagnetic radiation are broadly classified into the following classes:<ref name="em-spectrum"/>
# Gamma radiation
# X-ray radiation
# Ultraviolet radiation
# Visible radiation
# Infrared radiation
# Microwave radiation
# Radio waves
This classification goes in the increasing order of wavelength, which is characteristic of the type of radiation.<ref name="em-spectrum"/>
While the classification scheme is generally accurate, in reality there is often some overlap between neighboring types of electromagnetic energy. For example, SLF radio waves at 60&nbsp;Hz may be received and studied by astronomers, or may be ducted along wires as electric power, although the latter is, strictly speaking, not electromagnetic radiation at all (see [[near and far field]])
The distinction between X-rays and gamma rays is based on sources{{Citation needed|date=September 2011}}: gamma rays are the photons generated from [[nuclear decay]] or other nuclear and subnuclear/particle process, whereas X-rays are generated by [[electron]]ic transitions involving highly energetic inner atomic electrons. Generally, nuclear transitions are much more energetic than electronic transitions, so usually, gamma-rays are more energetic than X-rays, but exceptions exist. By analogy to electronic transitions, [[muonic atom]] transitions are also said to produce X-rays, even though their energy may exceed {{convert|6|MeV}},<ref>[http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-0335.pdf Corrections to muonic X-rays and a possible proton halo] slac-pub-0335 (1967)</ref> whereas there are many (77 known to be less than {{convert|10|keV|abbr=on}}) low-energy nuclear transitions (e.g. the {{convert|7.6|eV|abbr=on}} nuclear transition of [[thorium]]-229), and despite being one million-fold less energetic than some muonic X-rays, the emitted photons are still called gamma rays due to their nuclear origin.<ref>{{cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/ems3.html#c5 |title=Hyperphysics (see Gamma-Rays |publisher=Hyperphysics.phy-astr.gsu.edu |accessdate=2010-10-16}}</ref>

Also, the region of the spectrum of the particular electromagnetic radiation is [[reference frame]] dependent (on account of the [[Doppler shift]] for light) so EM radiation which one observer would say is in one region of the spectrum could appear to an observer moving at a substantial fraction of the speed of light with respect to the first to be in another part of the spectrum. For example, consider the [[cosmic microwave background]]. It was produced, when matter and radiation decoupled, by the de-excitation of hydrogen atoms to the ground state. These photons were from [[Lyman series]] transitions, putting them in the ultraviolet (UV) part of the electromagnetic spectrum. Now this radiation has undergone enough cosmological [[red shift]] to put it into the microwave region of the spectrum for observers moving slowly (compared to the speed of light) with respect to the cosmos. However, for particles moving near the speed of light, this radiation will be [[blue-shift]]ed in their rest frame. The highest energy cosmic ray protons are moving such that, in their rest frame, this radiation is blueshifted to high energy gamma rays which interact with the proton to produce bound quark-antiquark pairs ([[pions]]). This is the source of the [[GZK limit]].

===Radio frequency===
{{Main|Radio frequency|Radio spectrum|Radio waves}}
[[Radio]] waves generally are utilized by [[antenna (radio)|antennas]] of appropriate size (according to the principle of [[resonance]]), with wavelengths ranging from hundreds of meters to about one millimeter. They are used for transmission of data, via [[modulation]]. [[Television]], [[mobile phone]]s, [[wireless networking]] and [[amateur radio]] all use radio waves. The use of the radio spectrum is regulated by many governments through [[frequency allocation]].

Radio waves can be made to carry information by varying a combination of the amplitude, frequency and phase of the wave within a frequency band. When EM radiation impinges upon a [[Electrical conductor|conductor]], it couples to the conductor, travels along it, and [[radio frequency induction|induces]] an electric current on the surface of that conductor by exciting the electrons of the conducting material. This effect (the [[skin effect]]) is used in antennas. '''EM''' radiation may also cause certain molecules to absorb energy and thus to heat up, causing thermal effects and sometimes burns. This is exploited in [[microwave oven]]s.

===Microwaves===
{{Main|Microwaves}}
[[Image:Atmospheric electromagnetic opacity.svg|thumb|right|350px|Plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation.]]
The [[super high frequency]] ([[SHF]]) and [[extremely high frequency]] ([[EHF]]) of [[microwave]]s come after radio waves. Microwaves are waves which are typically short enough to employ tubular metal [[waveguide]]s of reasonable diameter. Microwave energy is produced with [[klystron]] and [[magnetron]] tubes, and with solid state [[diode]]s such as [[Gunn diode|Gunn]] and [[IMPATT diode|IMPATT]] devices. Microwaves are absorbed by molecules that have a [[Molecular dipole moment|dipole moment]] in liquids. In a [[microwave oven]], this effect is used to heat food. Low-intensity microwave radiation is used in [[Wi-Fi]], although this is at intensity levels unable to cause thermal heating.

Volumetric heating, as used by [[microwave oven]]s, transfers energy through the material electromagnetically, not as a thermal heat flux. The benefit of this is a more uniform heating and reduced heating time; microwaves can heat material in less than 1% of the time of conventional heating methods.

When active, the average microwave oven is powerful enough to cause interference at close range with poorly shielded electromagnetic fields such as those found in mobile medical devices and cheap consumer electronics.

====Terahertz radiation====
{{Main|Terahertz radiation}}

Terahertz radiation is a region of the spectrum between far infrared and microwaves. Until recently, the range was rarely studied and few sources existed for microwave energy at the high end of the band (sub-millimetre waves or so-called [[terahertz radiation|terahertz waves]]), but applications such as imaging and communications are now appearing. Scientists are also looking to apply terahertz technology in the armed forces, where high frequency waves might be directed at enemy troops to incapacitate their electronic equipment.<ref>{{cite news|title = Advanced weapon systems using lethal Short-pulse terahertz radiation from high-intensity-laser-produced plasmas|date = March 6, 2005|url = http://www.indiadaily.com/editorial/1803.asp|work = India Daily|accessdate = 2010-09-27}}</ref>

===Infrared radiation===
{{Main|Infrared radiation}}
The [[infrared]] part of the electromagnetic spectrum covers the range from roughly 300&nbsp;GHz (1&nbsp;mm) to 400 THz (750&nbsp;nm). It can be divided into three parts:<ref name="em-spectrum"/>
*'''Far-infrared''', from 300&nbsp;GHz (1&nbsp;mm) to 30 THz (10 μm). The lower part of this range may also be called microwaves. This radiation is typically absorbed by so-called rotational modes in gas-phase molecules, by molecular motions in liquids, and by [[phonons]] in solids. The water in the Earth's atmosphere absorbs so strongly in this range that it renders the atmosphere effectively opaque. However, there are certain wavelength ranges ("windows") within the opaque range which allow partial transmission, and can be used for astronomy. The wavelength range from approximately 200 μm up to a few mm is often referred to as [[Submillimetre astronomy|"sub-millimetre" in astronomy]], reserving far infrared for wavelengths below 200 μm.
*'''Mid-infrared''', from 30 to 120 THz (10 to 2.5 μm). Hot objects ([[black-body]] radiators) can radiate strongly in this range. It is absorbed by molecular vibrations, where the different atoms in a molecule vibrate around their equilibrium positions. This range is sometimes called the ''fingerprint region'' since the mid-infrared absorption spectrum of a compound is very specific for that compound.
*'''Near-infrared''', from 120 to 400 THz (2,500 to 750&nbsp;nm). Physical processes that are relevant for this range are similar to those for visible light.

===Visible radiation (light)===
{{Main|Visible spectrum}}
Above infrared in frequency comes [[visible light]]. This is the range in which the [[sun]] and other [[star]]s emit most of their radiation{{Citation needed|date=August 2011}} and the spectrum that the [[luminosity function|human eye is the most sensitive]] to. Visible light (and near-infrared light) is typically absorbed and emitted by electrons in molecules and atoms that move from one energy level to another. The light we see with our eyes is really a very small portion of the electromagnetic spectrum. A [[rainbow]] shows the optical (visible) part of the electromagnetic spectrum; infrared (if you could see it) would be located just beyond the red side of the rainbow with [[ultraviolet]] appearing just beyond the violet end.

Electromagnetic radiation with a [[wavelength]] between 380 [[nanometre|nm]] and 760&nbsp;nm (790–400 terahertz) is detected by the human eye and perceived as visible light. Other wavelengths, especially near infrared (longer than 760&nbsp;nm) and ultraviolet (shorter than 380&nbsp;nm) are also sometimes referred to as light, especially when the visibility to humans is not relevant. White light is a combination of lights of different wavelengths in the visible spectrum. Passing white light through a prism splits it up in to the several colors of light observed in the visible spectrum between 400 nm to 780 nm.

If radiation having a frequency in the visible region of the EM spectrum reflects off an object, say, a bowl of fruit, and then strikes our eyes, this results in our [[visual perception]] of the scene. Our brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this not-entirely-understood psychophysical phenomenon, most people perceive a bowl of fruit.

At most wavelengths, however, the information carried by electromagnetic radiation is not directly detected by human senses. Natural sources produce EM radiation across the spectrum, and our technology can also manipulate a broad range of wavelengths. [[Optical fiber]] transmits light which, although not necessarily in the visible part of the spectrum, can carry information. The modulation is similar to that used with radio waves.

===Ultraviolet light===
{{Main|Ultraviolet}}
[[Image:Ozone altitude UV graph.svg|right|thumb|230px|The amount of penetration of UV relative to altitude in Earth's [[ozone layer|ozone]]]]
Next in frequency comes [[ultraviolet]] (UV). The wavelength of UV rays is shorter than the violet end of the [[visible spectrum]] but longer than the X-ray.

Being very energetic, UV rays can break chemical bonds making molecules unusually reactive or ionizing them (see [[photoelectric effect]]) in general changing their physical behavior. [[Sunburn]], for example, is caused by the disruptive effects of UV radiation on [[Human skin|skin]] [[Cell (biology)|cells]], which is the main cause of [[skin cancer]]. UV rays can irreparably damage the complex [[DNA]] molecules in the cells producing [[thymine dimers]] making it a very potent [[mutagen]]. The sun emits a large amount of UV radiation, which could potentially turn Earth into a barren desert. However, most of it is absorbed by the atmosphere's [[ozone layer]] before it reaches the surface.

===X-rays===
{{Main|X-rays}}
After UV come [[X-ray]]s, which are also ionizing, but due to their higher energies they can also interact with matter by means of the [[Compton scattering|Compton effect]]. Hard X-rays have shorter wavelengths than soft X-rays. As they can pass through most substances, X-rays can be used to 'see through' objects, most notably diagnostic X-ray images in medicine (a process known as [[radiography]]), as well as for high-energy physics and astronomy. [[Neutron star]]s and accretion disks around [[black hole]]s emit X-rays, which enable us to study them. X-rays are given off by stars and are strongly emitted by some types of nebulae.

===Gamma rays===
{{Main|Gamma rays}}
After hard X-rays come [[gamma rays]], which were discovered by [[Paul Villard]] in 1900. These are the most energetic [[photons]], having no defined lower limit to their wavelength. They are useful to [[astronomy|astronomers]] in the study of high energy objects or regions, and find a use with physicists thanks to their penetrative ability and their production from [[radioisotopes]]. Gamma rays are also used for the [[irradiation]] of food and seed for sterilization, and in medicine they are used in [[Radiation oncology|radiation cancer therapy]] and some kinds of diagnostic imaging such as [[Positron emission tomography|PET scans]]. The wavelength of gamma rays can be measured with high accuracy by means of Compton scattering.

Note that there are no precisely defined boundaries between the bands of the electromagnetic spectrum. Radiation of some types have a mixture of the properties of those in two regions of the spectrum. For example, red light resembles infrared radiation in that it can [[resonance|resonate]] some [[chemical bonds]].

==See also==
{{Portal|Electronics}}
{{Colbegin|2}}
*[[Atmospheric window]]
*[[Bandplan]]
*[[Cosmic rays]]
*[[Electroencephalography]]
*[[Ionizing radiation]]
*[[List of international common standards]]
*[[Ozone layer]]
*[[Radiant energy]]
*[[Radiation]]
*[[Spectroscopy]]
*[[V band]]
*[[W band]]
{{Colend}}

==References==
{{Refimprove|date=November 2007}}
{{Reflist|2}}

==External links==
{{Commons category|Electromagnetic spectrum}}
*[http://www.unwantedemissions.com UnwantedEmissions.com] (U.S. radio spectrum allocations resource)
*[http://www.acma.gov.au/webwr/radcomm/frequency_planning/spectrum_plan/arsp-wc.pdf Australian Radiofrequency Spectrum Allocations Chart] (from Australian Communications and Media Authority)
*[http://www.ic.gc.ca/epic/site/smt-gst.nsf/vwapj/spectallocation-08.pdf/$FILE/spectallocation-08.pdf Canadian Table of Frequency Allocations] (from [[Industry Canada]])
*[http://www.ntia.doc.gov/osmhome/allochrt.html U.S. Frequency Allocation Chart] — Covering the range 3&nbsp;kHz to 300&nbsp;GHz (from [[United States Department of Commerce|Department of Commerce]])
*[http://www.ofcom.org.uk/static/archive/ra/topics/spectrum-strat/future/strat02/strategy02app_b.pdf UK frequency allocation table] (from [[Ofcom]], which inherited the [[Radiocommunications Agency]]'s duties, pdf format)
*[http://www.e-builds.com/EM%20spectrum/ Flash EM Spectrum Presentation / Tool] – Very complete and customizable.
*[http://mintaka.sdsu.edu/GF/explain/optics/rendering.html#CIEdiag/ How to render the color spectrum / Code] – Only approximately right.
*[http://unihedron.com/projects/spectrum/downloads/spectrum_20090210.pdf Poster "Electromagnetic Radiation Spectrum"] (992 kB)

{{EMSpectrum}}

[[Category:Electromagnetic spectrum| ]]
[[Category:Waves| ]]

[[af:Elektromagnetiese spektrum]]
[[ar:طيف كهرومغناطيسي]]
[[an:Espectro electromagnetico]]
[[ast:Espectru electromagnéticu]]
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[[bg:Електромагнитен спектър]]
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[[cs:Elektromagnetické spektrum]]
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[[de:Elektromagnetisches Spektrum]]
[[el:Ηλεκτρομαγνητικό φάσμα]]
[[es:Espectro electromagnético]]
[[eo:Elektromagneta spektro]]
[[eu:Espektro elektromagnetiko]]
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[[it:Spettro elettromagnetico]]
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[[uk:Електромагнітний спектр]]
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