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Homework Help: Determine dispersion from fitting equation

  1. Oct 2, 2012 #1
    1. The problem statement, all variables and given/known data

    Determine the dispersion (dn/d lambda) at a wavelength of 800 nm from fitting equation.

    2. Relevant equations

    Fitting equation from graph of n vs 1/lambda^2

    y = 7e-15 x + 1.60

    3. The attempt at a solution

    Since it is a plot of n vs 1/lambda^2 I thought that x would be 1/lambda^2 so I took the derivative of that to get -2/lambda^3 and then put in the number. However I did not get a correct answer consistent with earlier findings.

    What am I doing wrong?
  2. jcsd
  3. Oct 3, 2012 #2

    rude man

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    Homework Helper
    Gold Member

    Looks like you went dn/d(1/λ2) = (dn/dλ)(dλ/d(1/λ2)
    = (-2/λ3)dn/dλ
    which is what I would have done.

    That math has to be correct, so you probably erred somewhere along the line in trying to fit a new curve of n vs. λ. That curve of course should look close to a straight line ...

    How about giving us a few numbers you computed?
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