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B Questions about light waves

  1. Mar 19, 2017 #1
    Hiya,

    I've been reading up on EM waves and I feel I'm slowly starting to get it, but I'm not entirely sure if I do, so I was wondering if anyone could help so I can be sure I'm not making some false conclusions here. I can't seem to find any answers to the less certain parts here

    So as I understand it, these EM waves contain an oscillating electrical field part. One that has a certain direction (which seems to be polarity based? right?) and an amplitude that oscillates between positive and negative. So that an electron or proton that the light passes through would end up bobbing along with the wave, following the field. With electrons/protons basically bobbing in opposite ways because of their opposite charges. And electrons that are all at equal distance from the lightsource would all have to bob up/down exactly synchronized, because they're under the influence of equal electric fields. And at shorter wavelengths, the electrons end up bobbing up and down at a faster frequency, but don't bob around over greater distances. Is that correct?

    But then light is actually quantized too somehow, so that these effects on the electrons aren't actually smooth, but kind of jumpy, right? So in cases where there's few photons, the electrons might not actually do their synchronized bobbing, and could for example have one that gets accelerated up, but never gets accelerated down again because we ran out of photons. Is this correct? And also, since the bobbing generally seems smooth (is that even the case?), that must mean there tend to be many light quanta per period.
    As far as I understand, the amount of light quanta also doesn't depend on the frequency of the light, (but instead they just become more energetic), so that you on average you would actually have less photons per period in light that has a shorter wavelength. So theoretically on average, in more energetic light, the bobbing of electrons would be less smooth, right?

    Besides the electric part, there's also a magnetic part. Does that part of the wave even do anything except keep the wave going?

    Thanks for your help!
     
  2. jcsd
  3. Mar 19, 2017 #2

    mfb

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    As long as we can ignore quantum mechanics: yes.
    The distance depends on the intensity of the light, not just on the frequency.
    No. If quantum mechanics becomes relevant, classical descriptions for the motion of electrons become meaningless.
    Depends on the intensity. There is no fixed relation between photon number and frequency.
    It has an influence on moving charged objects.
     
  4. Mar 19, 2017 #3
    Thanks! That helps a lot

    Hmm, so is there anything that can still be said about the electrons? Do they even oscillate in quantum mechanics then?
     
  5. Mar 19, 2017 #4

    mfb

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    There are cases where you can have something like oscillations in the wave functions, yes.
     
  6. Mar 19, 2017 #5

    sophiecentaur

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    There are situations where electrons have a continuum of possible states and the motion of an individual electron can be treated individually. In a low density plasma, (in the Ionosphere, for instance) individual electrons can be considered to oscillate from side to side as a Radio Wave passes through. See this link. and search Appleton Hartree equation. The Quantum levels are so close together that it can be treated as a continuum. The positive ions are much more massive so their movement is not significant (Centre of mass doesn't move)
     
  7. Mar 21, 2017 #6
    So these possible positional states are transverse compared to the wave and the direction depends on the polarity right?
    Do you happen to know of any experiments that have shown this behavior?
     
  8. Mar 21, 2017 #7

    sophiecentaur

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    A familiar example of this motion of electrons would be the way an electron beam is scanned across the face of a TV screen. There is a 15kHz oscillating field that deflects electrons. In a Cathode Ray Oscilloscope the beam is also deflected in time with the applied signal (can be hundreds of MHz). The deflection field is Magnetic in one case and Electric in the other.
     
  9. Mar 21, 2017 #8
    Hmm, so is this oscillation of the field accomplished through light/photons? I had assumed these types of fields were created through other means such as running current through wires and that this didn't create light. Or is it actually the case that all EM field oscillations can be seen as light/photons?
     
  10. Mar 21, 2017 #9

    sophiecentaur

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    There is no difference, in principle, between EM waves of all frequencies but there are details in the way they are formed and their effects for the various frequencies involved. The physical rate of oscillation would be different and the actual deflection would be nonsensically small for a simple model of an electron being subject to a wave with light frequency. The mechanical idea does work for 1MHz radio waves, though.
    It is not meaningful to talk of the waves, when on the move through space, as consisting of photons (aka little bullets). Photons, as a concept, become relevant at either end of the journey because they are the quanta of energy that the wave can gain or lose.
     
  11. Mar 21, 2017 #10

    Nugatory

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    [Mentor's note]
    A number of posts based on a misunderstanding of the relationship between electromagnetic radiation as a classical theory and the photons that appear in quantum electrodynamics have been removed. All posters are asked to focus on Xilor's original question and followups.
     
  12. Mar 21, 2017 #11
    Let me rephrase my question: Does that mean that any EM field oscillation could end up doing what photons do at the ends of their journeys? Could say an oscillating electric field created by changing current in a wire, cause an electron in an atom to jump to a different energy state? And could it then actually be visible if it oscillates within the visible light spectrum frequencies? Or do these types of waves remain different in this regard and are only the same in their ability to accelerate charged particles through the way they affect the field?
     
  13. Mar 21, 2017 #12

    sophiecentaur

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    This means RF signals. The quantum energies are very low for these frequencies ( the very reason that they can be generated by oscillating charges in a wire) and the energy levels are in a continuous range and the quantum levels are lower than the natural thermal energy fluctuations. Electrons on the path of the wave can also interact with the charges in a 'classical' way.
    The energy levels involved with optical transitions are a million times higher than the energy transitions (quanta) involved at RF. An RF wave cannot interact with electrons in a bound state in an atom. You are hopping between QM and Classical and you need to be better acquainted with both ends before trying to discuss cases that are on the cusp in between.
     
  14. Mar 21, 2017 #13
    Current cannot oscillate fast enough to reach visible light like frequencies? Not even in a theoretical sense?

    That is probably good advice. I've sort of been trying to understand both simultaneously in some intuitive sense and that's probably not wise, especially not when QM can get so counterintuitive. I should definitely read up more
     
  15. Mar 21, 2017 #14

    mfb

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    It can, it leads to some weak scattering, an extreme case of Rayleigh scattering.
    It can (optical rectenna), and I don't see how you got the impression they could not.
     
  16. Mar 21, 2017 #15

    sophiecentaur

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    Is that interaction with the electron? Which quantum numbers would be involved? The first few transitions of an isolated atom lie in the optical region don't they? I know that's noddy level QM but there has to be some appropriate energy transition involved.
    The rectenna is essentially a receiving device, though. Its there a transmitting equivalent - to satisfy that question of Xilor's?
     
  17. Mar 22, 2017 #16

    mfb

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    You don't need a transition. You can treat the low-frequency electromagnetic wave as periodic perturbation of the states, leading to a time-dependent electric dipole moment which emits radiation at the same frequency.
    The same device can receive and emit light. In terms of practical applications, the receiving part is more interesting. The Wikipedia article discusses the emission as well.
     
  18. Mar 22, 2017 #17

    sophiecentaur

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    I guess it's not a (huge) problem to replace a rectifier with an LED.
     
  19. Mar 23, 2017 #18
    So just to confirm, if you manage to oscillate a current fast enough (setting practicality aside for a moment), you could create oscillations in the surrounding EM field that would manifest as waves with frequencies in the visible spectrum, and these waves would obtain energy somehow so that they could cause electron excitation and would thus be visible as light?
     
  20. Mar 23, 2017 #19

    sophiecentaur

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    If you are trying to think in terms of physical movement of charges, you need to bear in mind that the classical displacement at this sort of frequency is extremely small (much less than the diameter of an atom) and, as mfb puts it the Dipole Moment is what varies. That involves both an electron and a (relatively) massive positive nucleus. Nothing is whizzing about anywhere so the mechanical visualisation hardly applies.
     
  21. Mar 23, 2017 #20

    mfb

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    Yes.
    They don't "obtain energy somehow". There are no waves without energy where you could add energy. The energy is where the electromagnetic fields are.
    Those electromagnetic waves can excite electrons, and they would be visible to human eyes, yes.
     
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