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I Are EM waves reflected by inducing Hertzian dipoles?

  1. Mar 13, 2016 #1
    A comment a lab script for a recent experiment I did noted that the mechanism by which reflection of EM waves occurs is through the induction of Hertzian dipoles in a material.

    Having read up on Hertzian dipoles, I have found nothing which discusses them in the context of reflection. I read that a Hertzian dipole is essentially two small spherical conductors connected by a wire, with charge flowing periodically between them, or something that can be approximated by this. I find it very hard to understand how, for example, a plank of wood would support or even vaguely resemble such a system.

    To summarise, can someone explain how Hertzian dipoles are the mechanism by which EM waves are reflected, or point me to resources that explain this? What bit corresponds to the spheres and which bit to the wire?
     
  2. jcsd
  3. Mar 13, 2016 #2
    You must look at the molecular or atomic structure of the reflective material. The atom or molecule will have a dipole moment. That is, one end will be slightly more negative and the other end will be slightly more positive. The incident EM wave operates on the dipole by moving it. As it returns to its normal position it re-radiates the EM wave thus reflecting it.
     
  4. Mar 14, 2016 #3
    I don't think the atomic structure is relevant here because, for instance, in a metal we can consider it to be filled with a dense cloud of mobile electrons. The electrons will move under the influence of the incident electric field in a uniform way over whatever area is involved, and will produce re-radiation. There are, therefore, no localised dipoles involved. However, if wishing to analyse the radiation pattern of a reflector using antenna array theory, the surface may be divided into rectangles about half x quarter wavelength, and each of these may be considered to be a dipole radiator forming part of a large array.
     
  5. Mar 16, 2016 #4
    Yes, a metal has a very large number of unbound electrons which are available for collision with a photon. There are also many bound electrons. Thermal considerations determine the ratios. Any charge will be moved by an EM wave. On non dipole atomic structures, the two charges cancel out and no motion happens. A dipole atom or moleculre in an atomic lattice can collide with and re-emit photons from any of the charge centers.

    If only unbound electrons could interact with photons, poor conductors such as quartz should not reflect light well. The opposite is true.
     
  6. Mar 17, 2016 #5
    Is the last statement really true? Is quartz a good reflector? It is, after all, used for lenses. The relative permittivity determines the reflection factor, and most dielectrics have a relative permittivity of less than ten, so the reflection cannot be as good as a metal.
    Also, am I correct that a heavy charge, such as a positive ion, will not be moved easily an EM wave, but the associated electrons will respond easily?
     
  7. Mar 17, 2016 #6
    Quartz is a good reflector but not as good as metals.

    The incident EM wave will move positive charges bonded to the lattice. Since their effective masses are very large, dut to being locked in a lattice, their recoil velocity and distance is really small. It is essentially recoiless. Very similar to Mossbauer absorption.

    The free electrons, being unbound, will respond quickly. This is why an EM wave cannot penetrate more than a few microns into any metal.
     
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