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EM wave interacting with refelcting surface

  1. Apr 6, 2015 #1
    When trying to explain reflection through the EM treatment of light waves, how do we account for the fact that the electric/magnetic field of the incoming light would penetrate into the medium from which it is reflecting off of?

    Diagrams like these:


    show the 'reflection point' on the axis of propagation. But, the fields which propagate about that axis would penetrate into the other medium.

    From what I understand, EM waves are essentially coupled electric and magnetic fields that 'go on creating each other'...if one of the two gets distorted (the one in the plane of the normal and incoming light wave), wouldn't it destroy the light wave?
  2. jcsd
  3. Apr 7, 2015 #2


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    What do you mean by "get distorted"? Do you mean the decrease of the number of lines or dots in the reflected and transmitted wave in that picture? That's just for telling us that the amplitudes of the TE and TM components are reduced by virtue of the energy conservation. And as long as the fields in both region satisfy the wave equation, we will still have a wave.
  4. Apr 8, 2015 #3


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    I find the diagram confusing. The incoming energy is a mix of ordinary and extraordinary waves, not TM and TE waves. The ratio of Magnetic to Electric is always constant for the material in which it is travelling, so that will alter in the glass.
    I find the diagram slightly confusing. The incoming ray can be regarded as a mixture of two light waves - in the plane of the paper and at right angles to it. The two behave differently at the surface, so we can study them separately. Each wave has magnetic and electric fields in a fixed ratio. For the wave in glass, the ratio is different to that in air.
  5. Apr 8, 2015 #4


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    Why do you think that it's a mix of ordinary and extraordinary waves? That terms only arise in the propagation inside an anisotropic material.
  6. Apr 8, 2015 #5
    I am not talking about the specific diagram. I am speaking in general about how to analyse reflection as an electromagnetic phenomenon. The thing is that the propagating electric field should penetrate beyond the reflecting surface (for the TM wave), and this should cause a polarisation to occur. With this added polarisation, won't the time varying electric field 'as seen by the light wave' be different from when it normally propagates? Wouldn't this cause the wave to reflect unevenly back?
  7. Apr 8, 2015 #6

    Andy Resnick

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    Google 'frustrated total internal reflection" or "TIRF microscopy". The incident field does indeed penetrate the second medium (it's an evanescent field).
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