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Homework Help: How to determine M (Waves)

  1. Feb 3, 2012 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    I'm using this equation to find the thickness of the polymer.

    [(m+(1/2)) λ = 2nt] (where "n" is the index of refraction and "t" is the thickness )

    3. The attempt at a solution

    I just want to know how m is determined. I've learned everything else, but I missed the first day on which m value to use and it's always jsut slipped my mind whenever I'm in class or lab.

    Thank you for your time.
  2. jcsd
  3. Feb 3, 2012 #2


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    Homework Helper

    m is any integer, including zero.

  4. Feb 5, 2012 #3
    Thanks, but I'm still unsure as to why "0" is used in the solution instead of "1" or "2".
  5. Feb 6, 2012 #4


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    First: it is easier to make a thinner coating then a thick one.

    Second: The reflectance depends on the wavelength, this function is about the form R=A+Bcos(4piND/lambda) (N is the refractive index, D the thickness). If the reflectance is zero at a specific wavelength, it stays low in the broadest wavelength range when D has the smallest possible value.
    The figure shows the reflectance spectrum of two coatings, one 100 nm, the other 300 nm thick. Both have zero reflectance at 600 nm.


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