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I How is light created from charge Oscillation?

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  1. Jul 16, 2016 #1
    Light is an electromagnetic wave and to be so it needs an oscillating electric field, so only charges that undergoes harmonic motion shold be able to generate light is that right?

    And if that's right how can an electron that goes form an excited state to a lower one emit a photon? It wasn't harmonic motion...
    Can someone explain me please the way light is created?

    And if an electromagnetic field is not perturbed as a wave, I mean if an electron is just accelerated what happens, what is emits?
     
    Last edited: Jul 16, 2016
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  3. Jul 16, 2016 #2

    Garlic

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    Charged particles emit synchrotron radiation/bremsstrahlung when accelerated/decelerated.

    In this case, photons are just carrying the energy freed from the deexcitation.
     
  4. Jul 16, 2016 #3
    But... photons represent light, light is created when a charge oscillates, there is no oscillation here, I can't get this...
     
  5. Jul 16, 2016 #4

    Garlic

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    There is no need for an oscillation to create photons, you can think photons as packets of energy.
     
  6. Jul 16, 2016 #5

    Garlic

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    Let's wait for a physics student/professor to reply, as I don't think I can explain this as good as them. I don't want to give any unreliable informations to you.
     
  7. Jul 16, 2016 #6
    No. There are other ways that light (photons) can be generated by a charge; @Garlic was correct to note that bremsstrahlung and synchrotron radiation are two of those ways (they are not the same; synchrotron radiation and its close neighbor cyclotron radiation are technically both subtypes of bremsstrahlung), but forgot to point out that they occur in the presence of an EM field that deflects the charged particle and alters its momentum. In these cases the deflection of the particle causes it to lose kinetic energy, and this energy is transferred to the EM field which emits the energy as a photon. The reason this happens is because a stationary EM field cannot have momentum, but a photon can; to get rid of the momentum, the EM field must emit a photon to carry the momentum away and preserve momentum conservation.

    Sure. Actually it is harmonic motion; specifically net orbital motion (which is why fuzzy inaccurate portrayals of electrons in orbitals around a nucleus describe them like planetary orbits).

    When an electron in an orbital absorbs a photon, it moves to an excited orbital. This orbital has higher net energy and orbital angular momentum than the previous one. However, it is unstable; the electron can re-emit the photon, and return to the ground state, and it does so spontaneously with a probability distribution over time (it's more likely to emit the photon the longer you wait). This is different from all types of bremsstrahlung (though it's related to it somewhat).

    See above; I think this will explain it but you may have to ask some more questions to understand what I mean.
     
  8. Jul 16, 2016 #7
    Ok... so by what I understood a photon is not only associated with a sinusoidally oscillating electro magnetic field, but also other shapes of electro magnetic field

    And bremsstrahlung is one of those

    Is that Right?
     
  9. Jul 16, 2016 #8

    Dale

    Staff: Mentor

    EM waves. There is no need for harmonic motion.
     
  10. Jul 16, 2016 #9
    How can something that is not oscillating product a wave?
    And can the electromagnetic field have a non sinusoidally shape?
    Would it be still light?
     
  11. Jul 16, 2016 #10
    Not exactly. The photon itself is always associated with an oscillating EM field, in fact that's what a photon essentially is, but the field that requires its creation need not be oscillating. Charged particles that experience momentum changes without momentum transfer have a convenient way of accounting for the momentum lost by emitting a photon to carry the lost momentum away and preserve conservation of momentum; it's convenient because they have the electric charge to mediate the creation of photons. Conservation of momentum is a basic property of spacetime.
     
  12. Jul 16, 2016 #11
    It produces a photon; it's the photon that has the wave nature, not necessarily the thing it was produced by. Photons are energy and carry momentum, and provided their momentum and energy (and some other things) are accounted for, i.e. all the conservation laws are obeyed, they can be freely created and destroyed (or rather, emitted and absorbed).

    Yes, if by "sinusoid" you mean an exact sine wave that fluctuates with zero as its median. The electric and magnetic waveforms can have various angular relations to one another, and this is all described by polarization modes such as helical and elliptical modes. However this area gets extremely complicated very quickly if you don't have a very strong math background. You can get a pretty good idea by looking up polarization on Wikipedia and following the links to understand the difference between plane polarization and other modes. But it's gonna take you a while to dig through it; one of the staff or a professional or professor might have better sources to suggest to gain an understanding of polarization.
     
  13. Jul 16, 2016 #12
    OMG thanks now I get it ☺☺☺☺

    And those are photons having strange waves asssociated with then?
     
  14. Jul 16, 2016 #13
    Ha! I'm glad I could give you a Eureka Moment.

    Not quite sure what you mean by "strange." They need not be orthogonal, nor symmetric around zero (though they still have symmetries).
     
  15. Jul 16, 2016 #14
    Like these in the photo, for example the square shaped
     

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  16. Jul 16, 2016 #15
    Heh, well, actually all of those are superpositions of sine waves. For example, the square wave is actually a superposition of all the odd-numbered sine wave multiples of the fundamental frequency of the square wave, with descending amplitudes as the frequency increases. But that has nothing to do with polarization, nor with emission and absorption of photons.

    So the answer to your question is, no, a single photon cannot have those kinds of waveforms; but multiple photons traveling together can, and you can add them up and treat them as a single wave, and you'll get the same results as if you treated them all individually.
     
  17. Jul 16, 2016 #16
    Ok, thanks a lot ☺
     
  18. Jul 16, 2016 #17

    Dale

    Staff: Mentor

    You seem to be thinking of the school book illustrations of sinusoidal waves. A sinusoidal wave has particularly simple properties, so they are typically presented first. However, that in no way implies that all waves are sinusoidal. Waves can be brief pulses, step functions, ramps, and many other non oscillating shapes.

    In short, the assumption that it must be harmonic is a mistake.
     
  19. Jul 16, 2016 #18
    Cozma Alex,
    Here is something that I filed a note to myself on a few years ago: If you think in classical terms about what might happen as a physical particle (electron) drops from an orbit to a lower one perhaps it would not "instantly" stabilize in that orbit, but rather drop below the orbit slightly then rise up above the orbit. It would oscillate up and down like that until adequate energy was given off in the form of an electromagnetic wave.

    I rather like that idea, being a reductionist at heart, thought you might too.

    DC
     
  20. Jul 16, 2016 #19

    bhobba

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    Such 'pictures' are simply an aid to intuition and is not what is really going on. For example we have the phenomena of spontaneous emission which is related to the fact elections are coupled to the quantum EM field that permeates all space and hence not in a stationary state.

    Until you are able to study QM and later Quantum Field Theory don't take such pictures to heart.

    Thabks
    Bill
     
  21. Jul 16, 2016 #20

    bhobba

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    Its the phenomena of spontaneous emission which, as I explained above, is because its coupled to the quantum EM field its not in a stationary state. Exactly what that means and implies needs to wait until you study QM and later QFT, but the following isn't bad at getting a bit of an idea whats going on
    http://www.physics.usu.edu/torre/3700_Spring_2015/What_is_a_photon.pdf

    Thanks
    Bill
     
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