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Homework Help: Explain why long wave radio does not cut out in tunnel.

  1. Jan 12, 2012 #1
    1. The problem statement, all variables and given/known data
    Explain why if your car is tuned to FM, it cuts out when you enter a tunnel but doesn’t if you are tuned to long wave reception.
    (λ of FM: 3m, λ of long wave: 1500m)

    2. Relevant equations

    3. The attempt at a solution
    Well, what I think is that..
    As the λ of the long wave is much longer than the width of the entrance of the tunnel, there will be diffraction when the wave goes into the tunnel. Diffraction will cause destructive interference, and depending on the car's position there will be cut-out.
    Therefore, I think FM should be the one with no cut out in the tunnel (which is of course contrary to my personal experience).

    Please enlighten me. :shy:
  2. jcsd
  3. Jan 12, 2012 #2

    Simon Bridge

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    Science Advisor
    Homework Helper

    The situation for radio in tunnels is actually quite complex
    - so the queston is actually about the most recent topic in your course. If that is diffraction and interference, then you are likely giving the correct answer.

    However: for diffraction through an aperture, you need the opening to be smaller than the wavelength - which is what you have in both cases. So it is probably not quite right.
    ... has a widget to investigate slit-width vs wavelength in diffraction.

    Also - when you drive through the tunnel, do you get a radio signal fading in and out or is it just gone? (If it goes at all.)

    You can also think about what the tunnels are made out of and how the different wavelength radiowaves behave with those ematerials.
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