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Homework Help: Bragg's law and double-slit experiment

  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]\lambda=\frac{1240eVnm}{5eV}[/tex]

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

    [tex]1x240nm=2dsin5[/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

    Astronuc

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    Some references on interference and multiple slits.

    http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/mulslidi.html#c2

    http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/slits.html

    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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