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

Homework Help: Doppler problems.

  1. Mar 7, 2006 #1
    Okay, I don't need an answer . . . I just need to know how I would go about getting an answer.

    Here is the question, exactly worded as on my worksheet:

    "What is the up-doppler for a 600.0 nanometer laser scanning an innocent, law-abiding, safe-driving, tax-paying, completely innocent, God-fearing, innocent, motorcyclist closing this barely legal speed trap at a reasonable for conditions 198.00 km/hr? Use mega sig figs."

    I converted km/hr into m/s and got 55 m/s for the speed of the motorcycle. Now, for the up-doppler, would I take the 600 nanometer and multiply it by the velocity of sound over the quantity (velocity sound + velocity of source)?

    I am really confused.
    Last edited: Mar 7, 2006
  2. jcsd
  3. Mar 7, 2006 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    For electromagnet or optical sources the speed of sound is inmaterial. You would use the speed of light.
  4. Mar 7, 2006 #3
    I don't understand that though . . . we are doing the unit on sound, so how would the speed of light be applicable?
  5. Mar 7, 2006 #4


    User Avatar
    Homework Helper

    Light travels as a wave, similar to sound.
    You're doing a chapter on WAVES.
    The Doppler effect occurs for water waves and string waves, too.
    For most waves, it is important whether the SOURCE is moving, or whether the RECEIVER is moving.
    In your case, the motorcycle is moving, the police car is not.
    The motorcyclist receives waves of lambda, at a rate f = (c+v)/lambda.
    If you want to treat the REFLECTED waves (as the police want to do),
    the waves that the motorcyclist emits have lambda_back = (c+v)/f .
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook