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Homework Help: Doppler Effect Source Moving Away

  1. Oct 31, 2011 #1
    1. The problem statement, all variables and given/known data
    Superman has a blue light of wavelength 480nm. How fast must he fly away from you so that his light appears orange, with a wavelength of 600nm?

    2. Relevant equations

    3. The attempt at a solution
    I used the equation [itex]f'=\frac{f}{\left(1+\frac{v_s}{v}\right)}[/itex] and solved for [itex]v_s[/itex] and got my answer to be [itex]75\times 10^8[/itex] m/s but the answer key said [itex]57.0\times10^6 - 67\times10^6[/itex]m/s. What did I do wrong?
    Last edited: Oct 31, 2011
  2. jcsd
  3. Nov 1, 2011 #2
    Someone told me that the equation [itex] \lambda =\lambda_{0}\sqrt{{1-\beta}\over{1+\beta}} [/itex] give the correct answer (like the other thread). Why didn't the usual doppler effect equation work here? I'm confused.
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