1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    User Avatar

    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

    User Avatar

    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 =]
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Doppler shift Light Date
Relativistic doppler shift Sep 28, 2017
Doppler shift source velocity components Apr 13, 2017
Relativistic doppler shift Feb 15, 2017
Doppler Shift Light from another galaxy Dec 17, 2010
Doppler shift of light Oct 5, 2006