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Photon's charge and electric field

  1. Jan 21, 2009 #1
    Light is an electromagnetic wave. i.e. it has both electric and magnetic field. But photon has no electric charge. Then, how light has electric field?..something i am missing here...
     
  2. jcsd
  3. Jan 21, 2009 #2

    Hootenanny

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    You missed a few words out there, let me fill in the gaps for you: Light is an electromagnetic wave according to the electromagnetic theory (Maxwell's Theory) of light.

    The photon however, is a construct of Quantum theory (QM, QED, QCD, QFT). Two different models for light, one considers light as an EM wave, the other as a particle.
     
  4. Jan 21, 2009 #3

    jtbell

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    In the classical picture, an electromagnetic wave is produced by oscillating or accelerating electric charges somewhere. Those charges might be a few feet away (light bulb), several miles away (radio or TV station), or many light-years away (distant star). But you still have to have those oscillating charges.

    In the quantum picture, photons are also produced by oscillating or accelerating electric charges. Those charges might be a few feet away... [copy and paste from preceding paragraph :smile:]

    Photons do not "produce" electric and magnetic fields in the sense that you're thinking of. Photons (on the one hand) and electromagnetic waves (on the other hand) are alternate descriptions of electromagnetic radiation. The radiation ultimately comes from the same source: oscillating or accelerating charges, in both descriptions.
     
  5. Jan 21, 2009 #4
    So there is contradiction between two theories on the charge of light or photon?
     
  6. Jan 21, 2009 #5

    Hootenanny

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    Light doesn't have a charge. As jtbell explained above in the EM theory of light, light is simply an electromagnetic wave, which is basically a perturbation of the electromagnetic fields. Electromagnetic fields are produced by accelerating charges, but they do not posses charge themselves.

    Edit: As an analogy, consider a stationary, isolated point charge. This electric charge 'creates' an electric field, however we could not say that this field possesses charge would we?
     
  7. Jan 21, 2009 #6

    jtbell

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    They both agree that light itself (whether described as photons or as electromagnetic waves) does not have electric charge.

    Keep in mind that because these are alternate descriptions of electromagnetic radiation, you need to be very very very very very careful about mixing them together.

    The two descriptions correspond to each other in certain ways. For example, the energy carried by electromagnetic radiation can be calculated from the amplitude of the electric (or magnetic) wave in the classical picture; or from the number of photons in the quantum picture. So you can say that a certain number of photons per square meter per second, hitting a surface (a detector), corresponds to a certain electric field amplitude. However, it is not correct (as far as I know) to divide the second number by the first one, to get an "electric field per photon" and say that each individual photon has or produces that amount of electric field. When you get down to the level of individual photons, for example by reducing the intensity of a light source to a very very low level, I don't think it even makes sense to talk about a classical electric field.
     
  8. Jan 21, 2009 #7

    turin

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    spidey, I think the concept you're missing is the distinction between electrostatics and electrodynamics. This business regarding the distinction between the classical and quantum description is irrelevant. In electrostatics, i.e. Coulomb's Law and such, there cannot be an electric field without a charge to source it. However, this is no longer true in electrodynamics. In fact, mathematically speaking, there are such solutions to Maxwell's equations that don't require a source anywhere in the universe. Such solutions are the so-called free-field solutions, aka plane wave solutions. It is just a simple fact of electrodynamics that a changing magnetic field can induce an electric field, no charges necessary. Why the changing magnetic field is there in the first place is a chicken-egg issue.
     
  9. Jan 22, 2009 #8
    Many thanks mates..i got it...
     
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