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Homework Help: Double slit peak intensity

  1. Sep 28, 2011 #1
    Hey guys,

    I've just got a couple of questions about this prelab I need to do for the Double Slit experiment.

    And just to make sure this is clear (I have no idea whether the method used in this experiment is normal) we have a double slit and a slit blocker. The slit blocker is after the double slit that can be adjust so it blocks either, neither or both slits.

    Now for the first question I have to compare (through calculations not data) the peak intensity of when both slits are open to when only one slit is open.

    My thoughts are that when both slits are open the peak intensity would double simply because the two waves constructively interfere at their peaks to produce double the intensity. However this image: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/imgpho/muls2.gif from hyperphysics makes me think I'm wrong as it suggests the peak intensities are the same. Why am I wrong?

    Also there's another question asking for a qualitative observation of what happens when the slit blocker goes gradually from blocking neither slit to blocking one slit. But I completely forgot to do this (massive prac) so could you please also confirm my suspicions?

    I suspect that the peak would simply drop in intensity (though this may be proved wrong in my last question) and slowly gain a larger spread. As in, looking at that same link it slowly transforms from one to the other.

  2. jcsd
  3. Sep 28, 2011 #2
    Hi Kyle,

    So while searching for a diagram courtesy of Google, I found that the Wikipedia page actually had an image of exactly what you are looking for, and an explanation

    I do hate to use Wikipedia like this but, easy is good. Check the "Overview" section image and explanation, it confirms exactly what you expected.


    I believe the Hyper physics image labels the single diffraction pattern as an "envelope", this multiplies the diffraction pattern of the double slit giving it the same peaks, and dips as the single slit, plus many more. But the intensity is "not to scale".

    Regarding your second question, it will be complex as we usually use two equal sized slits and you are effectively changing the size of one slit. I would be interested to know the result!

    All the best
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