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Metal reflects light & Antenna absorbs light & cuircuit

  1. Mar 29, 2015 #1
    Hi Forum,

    Couple of pheromone here:

    Metal reflects EM waves (visible light and lower)
    Antenna absorbs EM waves
    electrical circuit reflects high frequency signals inside the circuit
    electrical wire with the right length and right signal becomes an open-circuit/Antenna

    I think my question is: What/why is reflection? from an electron's point of view, does circuit reflecting voltage and metal reflecting em waves have the same fundamental governing principle?
     
  2. jcsd
  3. Mar 29, 2015 #2

    davenn

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    never heard that before ... doesn't sound right, where did you hear that ?
     
  4. Mar 29, 2015 #3
    I think you got owned by spell check...

    To answer your question, yes. In both cases the electromagnetic wave moves electrons. The moving electrons radiate a new wave, that is, the reflected wave. In the case of a radio antenna / receiver the receiver absorbs most of the energy before it can be re-radiated.
     
  5. Mar 29, 2015 #4
    Thank you, so, what's the magic that decides absorption or reflection? I don't need to the exact formulas or magic numbers like quarter or half wavelength, but the underlying principle. Thanks
     
  6. Mar 30, 2015 #5
    To vastly oversimplify it, the underlying principle is impedance. All homogenous materials have a characteristic impedance (frequency-dependent)- that of free space is uniquely almost exactly 377 Ohms (notice- same unit as resistance) over the whole spectrum. When electromagnetic waves encounter the boundary between two media (air and metal, for instance) with a strong difference (or "mismatch" in antenna terminology) they are reflected *unless* the physical dimensions of the boundary allow the incoming photons to resonate, inducing currents in the metal, which is what absorption really is. The same principle allows EM to exit an antenna. Notice it's the same principle behind optics at visible frequencies.
     
  7. Mar 30, 2015 #6
    Thank you, I think that was a very interesting generalization, I will look into it.

    However, I was really hoping for a more electron-photon kind of explanation. vaguely I can recall that EM on Metal reflection is governed by some sort of calculation so that EM will be re-radiated from the Metal with 180 degrees phase shift. Could some one point me to a good internet link for that?
     
    Last edited: Mar 30, 2015
  8. Mar 30, 2015 #7

    sophiecentaur

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    If you do not load an antenna then no energy will be absorbed. - just as with a piece of metal There is no paradox or contradiction here because you are specifying two entirely different situations.
    If an incident wave causes charges to flow (electrons to 'vibrate', in a crude model) and there is no energy absorbed due to some resistance, an EM wave will be re-emitted. Some energy will be stored reactively in the 'structure'.
     
  9. Mar 30, 2015 #8
    Excellent, thanks. That makes more sense. So , how do we explain the 180 degree phase shift? could you point me to a good place with some mathematical formulations?
     
  10. Mar 31, 2015 #9

    sophiecentaur

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    The 180 degree phase shift is best explained by the Boundary Conditions on the surface of the (perfect) conductor. There can be no volt drop across the surface so the re radiated wave will be in anti phase to the incident wave. The currents across the surface will have the 'appropriate' phase to achieve this. The same argument applies for any wave hitting any surface (sound. ocean etc.).
    If you want to read more then I suggest you Google terms like em wave reflections and pick out a reference that suits your level of understanding. There are many different levels of presentation and you are the best person to choose an appropriate one.
     
  11. Apr 1, 2015 #10
    Thank you, I found it!
    notangential.gif
     
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