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Bragg's law

  1. Feb 22, 2006 #1
    I am asked to construct a double-slit experiment for 5eV electrons. The first minimum of the diffraction pattern needs to occur at 5 degrees, what must the seperation of slits be?

    I'm guessing that I should use Bragg's law but I dont know how to calcualte a minumum because bragg's law is for maximums.


    [tex]n \lambda=2dsin \theta[/tex]


    i would solve for d but wouldnt that be a maximum?

    Also, I am asked to determin how far the slits have to be from the detector plan if the first minima on each side of the central maximum are seperated by 1 cm.

    we know that theta is 5 and the height is 1 cm, so... using 1cm/theta(5)=11.4 cm? that doesnt seem right to me.

    any ideas as to what i am doing wrong?
  2. jcsd
  3. Feb 25, 2006 #2


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    Staff: Mentor

    Some references on interference and multiple slits.



    The maxima occur where the difference in distance is an integral multiple of wavelength, so that two peaks constructively interact. The minima occur where a peak and trough occur, or one wave is a half-wavelength out of phase with the other, so that they destructively interfere.

    So a minimum should occur approximately halfway between two adjacent maxima.
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