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A question on CD diffraction

  1. Oct 23, 2011 #1
    1. The problem statement, all variables and given/known data
    White light was shone vertically down on a cd tilted 5°, with adjacent tracks d = 720 nm. An observer some distance away observes a color of light. Determine that wavelength of light observed. (Refer to picture attachment)

    2. Relevant equations
    finding path difference

    3. The attempt at a solution

    I understand that finding the path difference is key to solving this problem. So, i found the path difference to be simply:

    d cos (90+ 5 - ∅) = (1)(λ)

    But the answer wrote path difference =
    d cos (90 + 5 - ∅) - d cos (90 - 5)

    Using the provided answer, it didnt make sense to me for the additional term of ' d cos (90 - 5)' to be there?

    Attached Files:

  2. jcsd
  3. Oct 23, 2011 #2


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    Gold Member

    I think there will be path length differences from the 5 degree tilt of the cd and from the angle of the viewer, see sketch below,

    Attached Files:

  4. Oct 24, 2011 #3
    hmm i still don't seem to be able to grasp the answer..
  5. Oct 24, 2011 #4


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    Gold Member

    Look at the sketch and follow the two paths. Calculate the path length difference from the first dashed line to the second dashed line. I think you will find the required answer.
  6. Oct 24, 2011 #5
    OH i get it! the tilting causes path difference before and after diffration at surface!

    the right beam has to go down more while the left beam has to travel more!
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