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Homework Help: Please help with diffraction grating question

  1. Apr 11, 2007 #1
    For a wavelength of 420 nm, a diffraction grating produces a bright fringe at an angle of 26°. For an unknown wavelength, the same grating produces a bright fringe at an angle of 56°. In both cases the bright fringes are of the same order m. What is the unknown wavelength?

    Info: The Diffraction Grating & Compact Discs, Digital Video Discs, and the Use of Interference

    My answers all keep coming out wrong. I don't know what I'm doing!
  2. jcsd
  3. Apr 12, 2007 #2

    Tom Mattson

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    It's hard for us to tell what you're doing wrong when you don't post your work. Let's see your attempted solution.
  4. Apr 20, 2007 #3
    According to the short cut formula
    n lambda = d sin theta
    just set up x sine angle to 420 sine angle ratio.
    However, this only works for small angles.
    If that does not give right answer, you have to distance ratios
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