Photon: Difference between revisions
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In [[physics]], a '''photon''' is an [[elementary particle]] associated with an [[electromagnetic wave]]. When an electromagnetic wave has wavelength λ then its frequency ν is given by ν = ''c''/λ, where ''c'' is the [[speed of light]] (≈ 3·10<sup>8</sup> m/s). A photon is a light quantum with energy ''E'' and momentum '''p''' with, | |||
:<math> | |||
E = h \nu, \quad \mathbf{p} = \hbar \mathbf{k},\quad\mathrm{and}\quad |\mathbf{k}| = \frac{2\pi}{\lambda}, | |||
</math> | |||
where ''h'' is [[Planck's constant]], ħ ≡ h/(2π) ≈ 1.055·10<sup>−34</sup> Js, and '''k''' is the [[wave vector]], a vector pointing in the direction of the propagation of the wave. Although a photon has linear momentum, it does ''not'' have [[rest mass]]. | |||
The first to see that the electromagnetic field consists of energy parcels (light quanta) was [[Albert Einstein]] in 1905.<ref>A. Einstein, ''Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt'' [On a heuristic point view regarding the creation and change of light], Annalen der Physik, vol. '''17''', pp. 132 - 148, | |||
[http://dx.doi.org/10.1002/andp.19053220607 online]. </ref> [[Max Planck]], five years earlier, assumed in his theory of [[blackbody radiation]] that a black body consists of material oscillators, and had made the revolutionary step that the energies of these oscillators are discrete, i.e., integral multiples of small energies—quanta. But Planck had not yet made the step to a quantized radiation field. | |||
In 1923 [[Arthur Compton]] scattered X-rays off electrons and showed that the light quanta have—in addition to energy—linear [[momentum]].<ref>A. Compton, '' A Quantum Theory of the Scattering of X-rays by Light Elements'', Physical Review, vol. '''21''', pp. 483 - 502 (1923) [http://dx.doi.org/10.1103/PhysRev.21.483 online] </ref> Three years later, in 1926, [[Gilbert N. Lewis]] proposed the name photon for a "particle of light".<ref>G. N. Lewis, ''The conservation of photons'', Nature vol. '''118''', pp. 874-875 [http://dx.doi.org/doi:10.1038/118874a0 online]</ref> This was the same year that [[Erwin Schrödinger]] proposed his [[Schrödinger equation|wave equation]], which formed the basis of the new [[quantum mechanics]]. It took another year before [[Paul A.M. Dirac]]<ref>P.A.M. Dirac, Proc. Royal Society (London), ''The Quantum Theory of the Emission and Absorption of Radiation'', vol. '''A114''', p. 243 (1927)</ref> was able to fit the concept of the light quantum in the framework of the new theory. | |||
Dirac quantized the EM radiation field, which means that he re-interpreted the classical electric and magnetic fields as quantum mechanical [[operator]]s with well-defined [[commutation relation]]s. The commutation relations are those of [[boson]]s, particles of integer spin. Photons have spin ''S'' = 1. The spin multiplicity 2''S''+1 = 3 is given by two components corresponding to the two [[polarization]] directions of the EM wave and the third spin component corresponding to the propagation direction '''k'''. | |||
==References== | |||
<references /> | |||
Revision as of 09:34, 28 August 2008
In physics, a photon is an elementary particle associated with an electromagnetic wave. When an electromagnetic wave has wavelength λ then its frequency ν is given by ν = c/λ, where c is the speed of light (≈ 3·108 m/s). A photon is a light quantum with energy E and momentum p with,
where h is Planck's constant, ħ ≡ h/(2π) ≈ 1.055·10−34 Js, and k is the wave vector, a vector pointing in the direction of the propagation of the wave. Although a photon has linear momentum, it does not have rest mass.
The first to see that the electromagnetic field consists of energy parcels (light quanta) was Albert Einstein in 1905.[1] Max Planck, five years earlier, assumed in his theory of blackbody radiation that a black body consists of material oscillators, and had made the revolutionary step that the energies of these oscillators are discrete, i.e., integral multiples of small energies—quanta. But Planck had not yet made the step to a quantized radiation field.
In 1923 Arthur Compton scattered X-rays off electrons and showed that the light quanta have—in addition to energy—linear momentum.[2] Three years later, in 1926, Gilbert N. Lewis proposed the name photon for a "particle of light".[3] This was the same year that Erwin Schrödinger proposed his wave equation, which formed the basis of the new quantum mechanics. It took another year before Paul A.M. Dirac[4] was able to fit the concept of the light quantum in the framework of the new theory.
Dirac quantized the EM radiation field, which means that he re-interpreted the classical electric and magnetic fields as quantum mechanical operators with well-defined commutation relations. The commutation relations are those of bosons, particles of integer spin. Photons have spin S = 1. The spin multiplicity 2S+1 = 3 is given by two components corresponding to the two polarization directions of the EM wave and the third spin component corresponding to the propagation direction k.
References
- ↑ A. Einstein, Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt [On a heuristic point view regarding the creation and change of light], Annalen der Physik, vol. 17, pp. 132 - 148, online.
- ↑ A. Compton, A Quantum Theory of the Scattering of X-rays by Light Elements, Physical Review, vol. 21, pp. 483 - 502 (1923) online
- ↑ G. N. Lewis, The conservation of photons, Nature vol. 118, pp. 874-875 online
- ↑ P.A.M. Dirac, Proc. Royal Society (London), The Quantum Theory of the Emission and Absorption of Radiation, vol. A114, p. 243 (1927)