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Homework Help: Double slit question

  1. Feb 10, 2010 #1
    1. The problem statement, all variables and given/known data

    A grating with 349 lines/mm is illuminated with light of wavelength 412 nm What is the angular separation between the two lines formed in order m=2?

    2. Relevant equations

    [tex]\vartheta[/tex] = m[tex]\lambda[/tex]/d

    3. The attempt at a solution

    I did not know how to use this equation successfully since I did not have d. I do not know what the 349 lines/mm has to do with anything, so that is obviously what I'm missing. Help!
  2. jcsd
  3. Feb 11, 2010 #2
    This is not a "double slit" question, it is a diffraction grating question, for which there is an appropriate equation. (Not the one you give)
    d is the separation between the lines in the grating and is calculated from the number of lines per mm that is given by a simple inverse relationship.
  4. Feb 11, 2010 #3


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    Staff Emeritus
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    That's okay, in diffraction problems the same equation is used for a diffraction grating and the double slit . (You can click on the terms "diffraction grating" and "double slit", in that previous sentence, for more info.)

    There are 349 line spacings in 1 mm. Therefore, the distance between 2 adjacent lines is ___?
  5. Feb 11, 2010 #4
    1/349! So that is equal to d? I worked the problem out for [tex]\vartheta[/tex] and I'm getting like .28, the multiple choice answers are 26, 40, and 33[tex]^{o}[/tex]...
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