Difference between revisions of "Photon"

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{{jargon}}
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A '''photon''' is a particle of light. The term was coined by the American physical chemist Gilbert Newton Lewis.<ref>{{cite web |title=Should creationists accept quantum mechanics?|author=Jonathan Sarfati|publisher=CMI|url=https://creation.com/creationists-quantum-mechanics#txtRef15|accessdate=August 1, 2013|quote=Einstein called this Lichtquant or light quantum, but the American physical chemist Gilbert Newton Lewis (1875–1946) coined the term photon which stuck}}</ref>
A '''photon'''  
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* is a [[fundamental particle]] belonging to a group of particles called the ''[[bosons]]'', which, according to the [[quantum field theory]], have [[integer spins]].
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* Photons mediate the [[electro-magnetic force]], which includes both [[magnetism]] and [[electricity]].
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When we "see", our eyes are receiving streams of photons reflected from the objects around us.
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Photons are [[bosons]], that is, particles that have integer [[Quantum field theory#Field Bosons Mediate Action At a Distance|spin]]. In [[quantum field theory]], photons are the mediators of the [[electromagnetism|electromagnetic force]].
  
In [[classical physics]] there was a disagreement about the fundamental nature of [[light]] - whether it was a [[particle]] or a [[wave]], as it seemed to exhibit the properties of both.  The accepted explanation now is that it is a particle, and that its wave-like properties arise from its lack of [[mass]] (so that it can distribute [[force]]s over long distances) and from the following [[interference]] phenomenon typical to all [[quantum particles]].
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==Historical development==
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Since Newton's publication of his Opticks in 1704, his corpuscle theory of light dominated the world of physics. However, this changed in the 19th century with experiments done by Augustin-Jean Fresnel and Thomas Young, which supported Christian Huygens's wave theory.  One of the proofs for this is that light experiences [[Optics#Physical optics|interference]] and [[Optics#Physical optics|diffraction]], which would not be displayed if light were a classical particle. Furthermore, [[James Clerk Maxwell]] theoretical prediction of electromagnetic waves, together with [[Hertz|Heinrich Hertz]] experiments led to conclude that light was an electromagnetic wave.
  
In [[quantum mechanics]], one can no longer say for sure what the outcome of an experiment will be.{{fact}} The only meaningful question is what the [[probability]] is that the experiment will produce a particular result. These probabilities are [[absolute squares]] of [[complex numbers]] called ''[[amplitudes]]'' associated to the possible outcomes of an experiment.
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However, a phenomenon called the [[photoelectric effect]] led to a different conclusion. The photoelectric is the emission of electrons from a metal which is illuminated with light. This by itself is in no contradiction to the wave theory of light, but certain peculiarities of it were. For example, it was observed that no electrons were emitted unless the frequency of light was greater than a threshold frequency, regardless of how intense the light beam was. Also, the energy of the emitted electrons was dependent only on the light frequency, not in its intensity. Since the energy of a wave depends on its amplitude, which in turn is related to its intensity, this was in contradiction with the wave theory of light.  
  
Now in [[classical physics]], if the outcome of an experiment could happen in 2 ways, with probabilities A and B, we would expect that the probability of this outcome would be A+B. However, in quantum mechanics, instead of adding the probabilities (that is, adding the squares of the amplitudes), we add the amplitudes first, and then square this to obtain the probability. These means that the amplitudes associated to the two possible outcomes can constructively or destructively interfere with each other, which gives rise to the wave-like behavior observed in quantum mechanics.
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[[Albert Einstein]] solved this problem arguing that light is really a particle, which was called a photon, each of which carry an energy given by ''h*f'', where ''h'' is [[Planck's constant]] and ''f'' is the frequency of the light. More intense light involves more photons, but the energy of each photon individually depends only on its frequency. This explained why there is a threshold frequency before a metal emits electrons: A single photon must be energetic enough to knock out an electron, and so increasing the intensity (sending more photons) will not solve the problem. Only increasing the frequency (increasing the energy per photon) will do.
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In standard interpretations of quantum mechanics, light behaves both as a wave and as a particle. The existence of photons is now well accepted by the physics community, and its implication has gone well beyond the photoelectric effect.  
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==Physics of photons==
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The Energy of a photon  depends on its frequency and is given by the equation:
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<math>E = hf</math>
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where ''f'' is the frequency of the electromagnetic wave and ''h'' is Planck's constant and has the numerical value of <math>6.626\times10^{-34}</math>Js ([[Joule]]s times second) or <math>4.135\times10^{-15}</math>eV s (electron volts times second).
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The [[momentum (physics)|momentum]] of a photon can be expressed as:
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<math>
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E = pc
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</math>
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==References==
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<references/>
  
 
==See also==
 
==See also==
*[[Interference]]
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*[[Optics#Physical optics|Interference]]
  
[[category:physics]]
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[[Category:Particle Physics]]

Latest revision as of 19:17, 9 April 2019

A photon is a particle of light. The term was coined by the American physical chemist Gilbert Newton Lewis.[1]

Photons are bosons, that is, particles that have integer spin. In quantum field theory, photons are the mediators of the electromagnetic force.

Historical development

Since Newton's publication of his Opticks in 1704, his corpuscle theory of light dominated the world of physics. However, this changed in the 19th century with experiments done by Augustin-Jean Fresnel and Thomas Young, which supported Christian Huygens's wave theory. One of the proofs for this is that light experiences interference and diffraction, which would not be displayed if light were a classical particle. Furthermore, James Clerk Maxwell theoretical prediction of electromagnetic waves, together with Heinrich Hertz experiments led to conclude that light was an electromagnetic wave.

However, a phenomenon called the photoelectric effect led to a different conclusion. The photoelectric is the emission of electrons from a metal which is illuminated with light. This by itself is in no contradiction to the wave theory of light, but certain peculiarities of it were. For example, it was observed that no electrons were emitted unless the frequency of light was greater than a threshold frequency, regardless of how intense the light beam was. Also, the energy of the emitted electrons was dependent only on the light frequency, not in its intensity. Since the energy of a wave depends on its amplitude, which in turn is related to its intensity, this was in contradiction with the wave theory of light.

Albert Einstein solved this problem arguing that light is really a particle, which was called a photon, each of which carry an energy given by h*f, where h is Planck's constant and f is the frequency of the light. More intense light involves more photons, but the energy of each photon individually depends only on its frequency. This explained why there is a threshold frequency before a metal emits electrons: A single photon must be energetic enough to knock out an electron, and so increasing the intensity (sending more photons) will not solve the problem. Only increasing the frequency (increasing the energy per photon) will do.

In standard interpretations of quantum mechanics, light behaves both as a wave and as a particle. The existence of photons is now well accepted by the physics community, and its implication has gone well beyond the photoelectric effect.

Physics of photons

The Energy of a photon depends on its frequency and is given by the equation:

where f is the frequency of the electromagnetic wave and h is Planck's constant and has the numerical value of Js (Joules times second) or eV s (electron volts times second). The momentum of a photon can be expressed as:

References

  1. Jonathan Sarfati. Should creationists accept quantum mechanics?. CMI. Retrieved on August 1, 2013. “Einstein called this Lichtquant or light quantum, but the American physical chemist Gilbert Newton Lewis (1875–1946) coined the term photon which stuck”

See also