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Light intensity and index of refraction

  1. Jan 21, 2015 #1
    Hi. Will the amplitude of Poynting's vector change if the electromagnetic wave goes from one medium to another?
    Shouldn't the amplitude remain constant due to conservation of energy? I.e. the photon-density and velocity will change, but their total intensity remains the same.

    I could always do the calculations from maxwell's equations, but it seems like too much bother right now if one of you guys already know this (I'm tired).

    Thanks
     
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  3. Jan 21, 2015 #2

    mfb

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    Why? If the wave propagation gets slower, the energy density goes up (because the flow stays constant if we can neglect losses).
     
  4. Jan 22, 2015 #3
    But if the wave propagation gets slower, less photons are going thru the surface in question.

    Btw, im talking about linear mediums only!
     
    Last edited: Jan 22, 2015
  5. Jan 22, 2015 #4

    mfb

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    No. Why do you think that?
    Yes I expected that.
     
  6. Jan 22, 2015 #5
    If the velocity of the photons is less, then fewer photons, per second, are passing thru the surface. So you could say that the photon flow gets "denser" if the light enters a medium with higher index of refraction than whence it came.

    Sorry, I neglected to add the per second bit
     
  7. Jan 23, 2015 #6

    mfb

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    No. All photons that go in will go out (this is exactly the "no absorption" assumption). If their speed gets lower, their density increases, but flow (here: photons per second) stays constant.
     
  8. Jan 23, 2015 #7
    Yes this is basically what I was saying with "doesn't the amplitude of Poynting's vector stay constant?".

    So the intensity remains unchanged, right?
     
  9. Jan 23, 2015 #8
    Didn't you imply that the intensity does not remain constant for the light beam, in the beginning?
     
  10. Jan 23, 2015 #9

    mfb

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    Power per area stays constant. I don't know how the Poynting vector is defined in a medium.
     
  11. Jan 24, 2015 #10

    blue_leaf77

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    There is always reflection. Reflected intensity + transmitted intensity = incoming intensity.
     
  12. Jan 29, 2015 #11
    Surely if the power per area stays constant, poynting's vector must stay constant?
     
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