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Doppler shift of Light

  1. Apr 11, 2009 #1
    1. The problem statement, all variables and given/known data
    How fast would a motorist have to be traveling for a yellow (l = 595 nm) traffic light to appear green (l = 550 nm) because of the Doppler shift?

    2. Relevant equations

    v=[(c)(f0/fs)2-c] / [(f0/fs)2+1]

    3. The attempt at a solution
    v= Speed of motorist
    c= Speed of light
    f0= Observed wavelength (green, (5.5x10^-7 m)
    fs= Source wavelength (yellow, (5.95x10^-7 m)

    First of all, am I using the correct equation? Because i'm almost certain my work is correct, since after plugging in the above known numbers, i get 2.35x10^7 m/s for an answer, but this answer is not correct. Any help would be greatly appreciated!
  2. jcsd
  3. Apr 12, 2009 #2

    Doc Al

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    Staff: Mentor

    Looks OK. (Except for some reason you use f to represent wavelength. f usually stands for frequency.)

    That answer looks good to me.
  4. Apr 12, 2009 #3
    oooh i love doppler shift =]

    ive always used F(observed)= [c/(c+v)]*F(actual)
  5. Apr 12, 2009 #4
    Thanks RoryP. I don't know what was wrong with the equation that i posted, but i tried yours out and it worked!
  6. Apr 12, 2009 #5

    Doc Al

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    Staff: Mentor

    That equation isn't quite right; it should be:

    [tex]f_{obs} = f \sqrt{\frac{c + v}{c - v}}[/tex]
  7. Apr 13, 2009 #6
    No worries Cheezay, yeah ive never seen the equation you started with, but then again ive only been doing physics for 2 years now so i might bump into it soon!

    Yeah i just checked my notes from 6th form and the equation i used is for c>>v, so dont konw if that makes any difference =]
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