Editing Electromagnetic radiation (section)

=== Particle model and quantum theory ===
{{See also|Quantization (physics)|Quantum optics}}

An anomaly arose in the late 19th century involving a contradiction between the wave theory of light and measurements of the electromagnetic spectra that were being emitted by thermal radiators known as [[black bodies]]. Physicists struggled with this problem unsuccessfully for many years, and it later became known as the [[ultraviolet catastrophe]]. In 1900, [[Max Planck]] developed a new theory of [[Planck's law of black-body radiation|black-body radiation]] that explained the observed spectrum. Planck's theory was based on the idea that black bodies emit light (and other electromagnetic radiation) only as discrete bundles or packets of energy. These packets were called [[quantum|quanta]]. In 1905, [[Albert Einstein]] proposed that light quanta be regarded as real particles. Later the particle of light was given the name [[photon]], to correspond with other particles being described around this time, such as the [[electron]] and [[proton]]. A photon has an energy, ''E'', proportional to its frequency, ''f'', by
: <math>E = hf = \frac{hc}{\lambda} \,\!</math>
where ''h'' is the [[Planck constant]], <math>\lambda</math> is the wavelength and ''c'' is the [[speed of light]]. This is sometimes known as the [[Planck–Einstein equation]].<ref>{{cite book
 | title = Physical Chemistry
 | author = Paul M. S. Monk
 | publisher = John Wiley and Sons
 | year = 2004
 | isbn = 978-0-471-49180-4
 | page = [https://archive.org/details/physicalchemistr00monk_857/page/n460 435]
 | url =https://archive.org/details/physicalchemistr00monk_857
 | url-access = limited
 }}</ref> In quantum theory (see [[first quantization]]) the energy of the photons is thus directly proportional to the frequency of the EMR wave.<ref>{{cite book
 |last=Weinberg
 |first=S.
 |author-link=Steven Weinberg
 |title=The Quantum Theory of Fields
 |volume=1
 |publisher=[[Cambridge University Press]]
 |year=1995
 |isbn=978-0-521-55001-7
 |pages=[https://archive.org/details/quantumtheoryoff00stev/page/15 15–17]
 |url=https://archive.org/details/quantumtheoryoff00stev/page/15
 }}</ref> Likewise, the momentum ''p'' of a photon is also proportional to its frequency and inversely proportional to its wavelength:
: <math>p = { E \over c } = { hf \over c } = { h \over \lambda }. </math>

The source of Einstein's proposal that light was composed of particles (or could act as particles in some circumstances) was an experimental anomaly not explained by the wave theory: the [[photoelectric effect]], in which light striking a metal surface ejected electrons from the surface, causing an [[electric current]] to flow across an applied [[voltage]]. Experimental measurements demonstrated that the energy of individual ejected electrons was proportional to the ''[[frequency]]'', rather than the ''[[intensity (physics)|intensity]]'', of the light. Furthermore, below a certain minimum frequency, which depended on the particular metal, no current would flow regardless of the intensity. These observations appeared to contradict the wave theory, and for years physicists tried to find an explanation. In 1905, Einstein explained this phenomenon by resurrecting the particle theory of light. Because of the preponderance of evidence in favor of the wave theory, however, Einstein's ideas were met initially with great skepticism among established physicists. Eventually Einstein's explanation was accepted as new particle-like behavior of light was observed, such as the [[Compton effect]].<ref name="Commins QM">{{cite book |last1=Commins |first1=Eugene |title=Quantum Mechanics; An Experimentalist's Approach |date=2014 |publisher=Cambridge University Press |isbn=978-1-107-06399-0}}</ref><ref>{{Cite book|last1=Ling|first1=Samuel J.|title=University physics. Volume 3|last2=Sanny|first2=Jeff|last3=Moebs|first3=William|publisher=OpenStax|year=2016|isbn=978-1-947172-22-7|chapter=The Compton Effect}}</ref>

As a photon is absorbed by an [[atom]], it [[excites]] the atom, elevating an electron to a higher [[energy level]] (one that is on average farther from the nucleus). When an electron in an excited molecule or atom descends to a lower energy level, it emits a photon of light at a frequency corresponding to the energy difference. Since the energy levels of electrons in atoms are discrete, each element and each molecule emits and absorbs its own characteristic frequencies. Immediate photon emission is called [[fluorescence]], a type of [[photoluminescence]]. An example is visible light emitted from fluorescent paints, in response to ultraviolet ([[blacklight]]). Many other fluorescent emissions are known in spectral bands other than visible light. Delayed emission is called [[phosphorescence]].<ref>{{Cite web|url=http://www.majordifferences.com/2016/11/7-differences-between-fluorescence-and-Phosphorescence.html|title=7 Differences between Fluorescence and Phosphorescence|last=Haneef|first=Deena T. Kochunni, Jazir|access-date=4 September 2017|url-status=live|archive-url=https://web.archive.org/web/20170904152324/http://www.majordifferences.com/2016/11/7-differences-between-fluorescence-and-Phosphorescence.html|archive-date=4 September 2017}}</ref><ref>{{Cite book|url={{google books |plainurl=y |id=kAn4AgAAQBAJ|page=93}}|title=Fundamental Physics of Radiology|last1=Meredith|first1=W. J.|last2=Massey|first2=J. B.|date=22 October 2013|publisher=Butterworth-Heinemann|isbn=978-1-4832-8435-4|language=en}}</ref>

Quantum mechanics also governs [[Emission (electromagnetic radiation)|emission]], which is seen when an emitting gas glows due to excitation of the atoms from any mechanism, including heat. As electrons descend to lower energy levels, a spectrum is emitted that represents the jumps between the energy levels of the electrons, but lines are seen because again emission happens only at particular energies after excitation.<ref>Browne, p 376: "Radiation is emitted or absorbed only when the electron jumps from one orbit to the other, and the frequency of radiation depends only upon on the energies of the electron in the initial and final orbits.</ref> An example is the emission spectrum of [[nebula]]e.<ref>{{cite book |last1=Hunter |first1=Tim B. |title=The Barnard Objects: Then and Now |last2=Dobek |first2=Gerald O. |date=19 July 2023 |publisher=Springer Cham |isbn=978-3-031-31485-8 |chapter=Nebulae: An Overview |bibcode=2023botn.book.....H |doi=10.1007/978-3-031-31485-8}}</ref> Rapidly moving electrons are most sharply accelerated when they encounter a region of force, so they are responsible for producing much of the highest frequency electromagnetic radiation observed in nature. These phenomena can be used to detect the composition of gases lit from behind ([[Absorption spectroscopy|absorption spectra]]) and for glowing gases ([[Emission spectrum|emission spectra]]). [[Spectroscopy]] (for example) determines what [[chemical element]]s comprise a particular star. Shifts in the frequency of the spectral lines for an element, called a [[redshift]], can be used to determine the star's [[Comoving and proper distances|cosmological distance]].<ref>{{Cite book |last=Longair |first=Malcolm S. |url=https://link.springer.com/10.1007/978-3-662-65891-8 |title=Galaxy Formation |date=2023 |publisher=Springer Berlin Heidelberg |isbn=978-3-662-65890-1 |series=Astronomy and Astrophysics Library |location=Berlin, Heidelberg |language=en |bibcode=2023gafo.book.....L |doi=10.1007/978-3-662-65891-8}}</ref>{{rp|181}}