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Double slit experiment problem help!

  1. Apr 2, 2013 #1
    1. The problem statement, all variables and given/known data

    Two narrow slits are .6 mm apart. The dark fringe of order 1 is 1.7 degrees from the central bright fringe. What is the wavelength of the light?

    2. Relevant equations

    D sinθ = (m + 1/2)λ --> chosen because it was destructive
    m= order
    d= distance between slit

    3. The attempt at a solution

    .6 (sin 1.7) = (1.5)λ
    (6*10^-4)(sin1.7) /(1.5)=λ

    I get , in nm ( after multiplying the answer I get 1*10^9) and I get an answer of 396666 nm. I plugged it in a few times in my calculator and I keep getting that answer. The website keeps saying it is wrong too, so I'm kinda lost
     
    Last edited: Apr 2, 2013
  2. jcsd
  3. Apr 2, 2013 #2

    PeterO

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    I think the formula is D sinθ = (m - 1/2)λ

    EDIT: Plus is the angle 1.7 or 1.17 degrees?
     
  4. Apr 2, 2013 #3

    rude man

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    Well, what can m range over? The first dark fringe is the 1st-order fringe.

    Plus, I ran your numbers, wrong though they are, and got a very different number for lambda.
     
  5. Apr 2, 2013 #4

    rude man

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    Depends on the range of m ...
     
  6. Apr 2, 2013 #5
    Well I don't think it is ( I was re-checking my notes and book ) but even if it was the answer is still wrong so I'm messing up somewhere :/
     
  7. Apr 2, 2013 #6

    PeterO

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    Did you notice my edit? is the angle 1.7 or 1.17 degrees?
     
  8. Apr 2, 2013 #7
    Well I just ran it again and, again, I got 3.97 * 10^5 nm again ( or 3.97 *^10^-4 m)
     
  9. Apr 2, 2013 #8
    Oh sorry it is 1.7 oops, I plugged in 1.7 in my calculator.I must've done a typo I'm sorry!
     
  10. Apr 2, 2013 #9

    PeterO

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    Is that correct or incorrect ?

    What happens if you you use your formula and m = 0 ?

    What happens if you use my formula with m = 1 ?
     
  11. Apr 2, 2013 #10

    PeterO

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    Is your calculator set to radians or degrees ? [just grasping at straws]
     
  12. Apr 2, 2013 #11
    Radians :).

    Plus if I use my formula with 0, I would get (6*10^-4)(sin 1.7)/ .5 = 1.19 *10^-3 m --> 1.19*10^-6 nm ( still wrong) and using your formula I get... the same thing because 1- 1/2 gives me .5 still... no?
     
  13. Apr 2, 2013 #12

    rude man

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    Time to replace the battery in your calculator? The expression above is correct - both of them!
     
  14. Apr 2, 2013 #13

    PeterO

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    Well if your calculator is set to radians, how are you hoping to get the sin of 1.7 degrees ?
     
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