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B Electron diffraction experiment puzzle

  1. Dec 7, 2016 #1
    In classical Physics wave theory (GCSE level) we talk about waves diffracting through a gap if the gap is similar size to (or smaller than) the wavelength of the waves.

    When firing fast electrons at a carbon target (teltron tube A level type apparatus) is it sufficient to say that if the de Broglie wavelength of the electrons (h/momentum) is similar to the interatomic spacing ( so the 'gaps') then we get good diffraction?

    If this interpretation is acceptable (ish) then my question is that if you slow down the electrons then their deBroglie wavelength increases (smaller momentum) but then they should still then show good diffraction because good diffraction occurs if wavelength is similar to (or larger than) the gap the 'wave' is passing through.

    If indeed slow electrons do show good diffraction then why the need for high voltages to accelerate the electrons to such a high speed using large voltages?

    Would love to have the kit myself to play with, unfortunately not!!

    Appreciate any thoughts.
  2. jcsd
  3. Dec 7, 2016 #2


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    The smaller the wavelength, the better the resolution you will get for whatever purpose you need those electrons for. And example will be an electron microscope.

  4. Dec 8, 2016 #3
    I found an equation that suggested that the radius of the interference rings follow 1/sqrt(electron velocity) so is it that with a higher voltage and faster electron you are more likely to be able to observe the rings on the fluorescent screen (and possibly more rings)?
  5. Dec 8, 2016 #4
    Have you come across the equation that gives the separation between the maxima in terms of the wavelength?
  6. Dec 9, 2016 #5
    Hi Jilang, yes I have. That's where the D proportional to 1/sqrt(v) came from.
  7. Dec 9, 2016 #6
    Then you have it sorted out. A slow electron will diffract well, but if it is too slow the maxima will be too spread out to observe.
  8. Dec 10, 2016 #7
    Hi ,
    That's great thanks. So I should definitely think of at as the electron waves interfering after reflecting off Brag planes in the microcrystals rather than the electron waves diffracting through the inter atomic gaps?
  9. Dec 10, 2016 #8
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