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Linewidths - Frequency and Wavelength

  1. Sep 2, 2014 #1

    Probably a really stupid question... but I'm confused about how to relate a line width in frequency to one in wavelength. To me it seems obvious that if there's a broader spread of frequencies, there must be a broader spread of wavelengths, and vice versa - after all, the line is broader.

    But... if you relate Δf and Δλ mathematically (f is frequency, since there was no nu on the quick symbols list):

    c = fλ

    f = c/λ

    df/dλ = -c/(λ^2)

    so df = dλ * (-c/(λ^2))

    I.e. if df is positive, dλ is negative...

    What am I doing wrong??
  2. jcsd
  3. Sep 2, 2014 #2


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Where's the problem in that? One slope is positive and the other is negative - that's what you get when you have

    λ= c/f and you write down dλ/df

    No worries. Have faith in yourself and your maths.
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