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Homework Help: Coherence length and coherence time of light

  1. Nov 18, 2009 #1
    1. The problem statement, all variables and given/known data

    Monochromatic light (λ = 500 nm) from a particular light source has a frequency
    bandwidth Δν = 6 x 1011 Hz.

    (a) What is the coherence length Δx and the coherence time Δt of this light?
    (b) What is approximately the largest optical path length difference in an
    interference experiment with this light, which would still give you observable
    interference fringes? Make a sketch of such an experiment.

    2. Relevant equations

    Coherence length: Δx= cΔt
    Path Difference: Δ=mλ=asinθ

    3. The attempt at a solution

    So, I figured out that Δx=5*10^-4 m and Δt=1.66*10^-12 s. I don't really know how to use these to find the largest optical path length. I have the equation, λ... What else do I need to figure out. Would I be using Young's Double Slit Experiment? Thanks for the help. :)
  2. jcsd
  3. Nov 19, 2009 #2


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    Homework Helper

    Re: Interference

    I don't think your numbers line up with the equations you've given... looks like a simple error, but i would check them, shouldn't you calculate Δt first?

    for the second part, think about splitting the beam of light in two then recombining... what do think will happen if the path length is greater than the coherence length

    Though it could apply , I don't think Young's double slit experiment is the best answer here, as it requires the light through each slit is coherent with each other, which happpens by having the same source for both slits, though the effect is probably more noticeable with the limited frequency content.
    Last edited: Nov 19, 2009
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