Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Doppler Effect Source Moving Closer

  1. Oct 31, 2011 #1
    1. The problem statement, all variables and given/known data
    Superman is carrying a red lantern with wavelength 650nm. He flies toward you at a speed of 2.7x108 m/s.
    What is the observed wavelength?

    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 got my answer to be [itex]65[/itex] nm. What did I do wrong?
    Last edited: Oct 31, 2011
  2. jcsd
  3. Oct 31, 2011 #2


    User Avatar
    Gold Member

    Well, for starters, the result you provided is a velocity (m/s), when in fact, what was asked for was a distance(m) (of a wavelength).
  4. Oct 31, 2011 #3
    OOPS, I accidentally copied the wrong thing. I actualy got 65 nm for the wavelength using the correct formula with the minus sign in the denominator, not plus sign (edited original post). Sorry about that.
  5. Oct 31, 2011 #4


    User Avatar

    Staff: Mentor

    So can you now do your other question thread for the source moving away?


  6. Oct 31, 2011 #5
    Bleh, I also forgot to mention that the answer key said 149nm.

    Sorry for not giving all the information, I made this all a big mess. I don't understand why the answer is 149nm, I'm pretty sure all my calculations were correct. I also forgot to mention that my answer was not what the answer key said in my other thread. What answer are you getting?
  7. Nov 1, 2011 #6
    Someone told me that the equation [itex] \lambda =\lambda_{0}\sqrt{{1-\beta}\over{1+\beta}} [/itex] give the correct answer. Why didn't the usual doppler effect equation work here? I'm confused.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook