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Homework Help: A screen with three slits and 1 gets blocked --> diffraction?

  1. May 3, 2016 #1
    1. The problem statement, all variables and given/known data
    A screen with three equally spaced slits of the width b and separation a is illuminated at normal incidence with monochromatic light with wavelength lambda. A diffraction pattern is obtained at a distance L on another screen positioned behind the screen with the slits. Calculate and compare the distance between the diffraction maxima with numbers m and m+1 for the cases when either the middle or one of the side slits is blocked. Assume a>>m*lambda

    2. Relevant equations
    3. The attempt at a solution
    Im not entirely sure what it is asking me to do but I started by finding the equations I wanted to use then figured the separation for blocking the middle would be 2a+b and the separation for blocking one of the side slits would just be a.

    Then I wrote the two cases up and took the difference between the Y at m=2 and m=1 for both. With no values given I'm not sure where to go next just with algebra, I subbed in some values of
    a=300 micron b = 10micron L = 10m lambda = 500nm
    and got 0.0082m distance in the first case and 0.017m in the second case but Im not sure this is what I'm being asked to do.

  2. jcsd
  3. May 3, 2016 #2


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    That's not logical if you use a for the side slit blocked case. Usually separation a means from center to center.
    Otherwise Ithink you are doing fine. Check with Hyperphysics if in doubt
  4. May 4, 2016 #3
    its just 2a, right? not 2a+b for when the middle slit closes. that way everything should scale and the y position of the middle slit blocking will be half
  5. May 5, 2016 #4


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    piece of cake. looks good to me
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