Editing Electromagnetic spectrum (section)

== Range ==
Electromagnetic waves are typically described by any of the following three physical properties: the [[frequency]] ''f'', [[wavelength]] [[lambda|λ]], or [[photon energy]] ''E''. Frequencies observed in astronomy 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 on the opposite end of the spectrum can be indefinitely long. Photon energy is directly proportional to the wave frequency, so gamma ray photons have the highest energy (around a billion [[electron volt]]s), while radio wave photons have very low energy (around a [[femto]]electronvolt). 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'' is the [[speed of light]] in vacuum
* ''h'' is the [[Planck constant]].

Whenever electromagnetic waves travel in a [[Transmission medium|medium]] with [[matter]], their wavelength is decreased. Wavelengths of electromagnetic radiation, whatever medium they are traveling through, are usually quoted in terms of the ''vacuum wavelength'', although this is not always explicitly stated.

Generally, electromagnetic radiation is classified by wavelength into [[radio wave]], [[microwave]], [[infrared]], [[visible light]], [[ultraviolet]], [[X-ray]]s and [[gamma ray]]s. The behavior of EM radiation depends on its wavelength. When EM radiation interacts with single atoms and [[molecule]]s, its behavior 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 wavelength range of 400&nbsp;[[Nanometre|nm]] to 700&nbsp;nm in a vacuum. A common laboratory spectroscope can detect wavelengths from 2&nbsp;nm to 2500&nbsp;nm.<ref name="em-spectrum" /> 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]] atoms [[Emission (electromagnetic radiation)|emit]] a [[radio wave]] photon that 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|author1=Condon, J. J.|author2=Ransom, S. M.|publisher=[[National Radio Astronomy Observatory]]|access-date=2008-01-05|archive-date=2011-05-04|archive-url=https://web.archive.org/web/20110504064425/http://www.cv.nrao.edu/course/astr534/Pulsars.html|url-status=dead}}</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|arxiv=astro-ph/0611691| title = Discovery of TeV Gamma-Ray Emission from the Cygnus Region of the Galaxy| journal = The Astrophysical Journal| volume = 658|issue=1| pages = L33–L36| year = 2007| last1 = Abdo | first1 = A. A.| last2 = Allen | first2 = B.| last3 = Berley | first3 = D.| last4 = Blaufuss | first4 = E.| last5 = Casanova | first5 = S.| last6 = Chen | first6 = C.| last7 = Coyne | first7 = D. G.| last8 = Delay | first8 = R. S.| last9 = Dingus | first9 = B. L.|author9-link= Brenda Dingus | last10 = Ellsworth | first10 = R. W.| last11 = Fleysher | first11 = L.| last12 = Fleysher | first12 = R.| last13 = Gebauer | first13 = I.| last14 = Gonzalez | first14 = M. M.| last15 = Goodman | first15 = J. A.| last16 = Hays | first16 = E.| last17 = Hoffman | first17 = C. M.| last18 = Kolterman | first18 = B. E.| last19 = Kelley | first19 = L. A.| last20 = Lansdell | first20 = C. P.| last21 = Linnemann | first21 = J. T.| last22 = McEnery | first22 = J. E.| last23 = Mincer | first23 = A. I.| last24 = Moskalenko | first24 = I. V.| last25 = Nemethy | first25 = P.| last26 = Noyes | first26 = D.| last27 = Ryan | first27 = J. M.| last28 = Samuelson | first28 = F. W.| last29 = Saz Parkinson | first29 = P. M.| last30 = Schneider | first30 = M.| display-authors = 29| bibcode = 2007ApJ...658L..33A|s2cid=17886934}}</ref>