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Homework Help: How to find slit separation?

  1. May 16, 2009 #1
    1. The problem statement, all variables and given/known data

    The green line of gaseous mercury at 546nm fals on a double-slit apparatus.
    If the fifth dark fringe is at 0.150 degree from the centerline, what is the slit separaton?

    2. Relevant equations

    d sin (theta) = m* lambda

    3. The attempt at a solution

    ok i used the above equatiob to find the d, slit separation
    as d=m*lambda/sin(theta)

    but im getting the wrong answer....and im not able to figure out the problem....plez
    som1 help.....!!
  2. jcsd
  3. May 17, 2009 #2


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    Homework Helper

    Hi patelpalak! :smile:

    (have a lambda: λ and a theta: θ :wink:)

    Show us your full calculations, and then we can see what went wrong, and we'll know how to help! :smile:
  4. May 17, 2009 #3
    ok here's what i did

    d sin (theta) =m*lamdba

    therefore, d=m*lambda/ sin(theta)

    m=5, since its fifth fringe
    lambda= 546*10^-9
    theta = 0.150 degrees

    so, d=(5)(546*10^-9)/sin(0.150)

    and i get 0.001042 meters
    but it gives me wrong answer....!!!!
    thats all i did ...
  5. May 18, 2009 #4


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    Hi patelpalak! :smile:

    (whatever happened to that λ and θ I gave you? :confused:)
    erm :redface: … θ has to be in radians! :wink:
  6. May 18, 2009 #5
    Hmmm.... If it is a dark fringe, isn't the equation something like:

    dsinθ = (m+0.5)λ


    Also, if my memory serves me correctly, you need to be careful with the order of your minimum. The first minimum has an m value of 0.
    Last edited: May 18, 2009
  7. May 19, 2009 #6
    ok thanx all i got the answer....
    appriciate your help....thanx once again!!:smile:
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