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Homework Help: The Doppler Effect for Light Waves

  1. Mar 29, 2012 #1
    1. The problem statement, all variables and given/known data
    You are cruising to Jupiter at the posted speed limit of 0.1c when suddenly a daredevil passes you, going in the same direction, at 0.3c. At what wavelength does your rocket cruiser's light detecter "see" his red tail lights? Is this wavelength ultraviolet, visible, or infrared? Use 650 nm for the wavelength of red light.

    2. Relevant equations
    I used the dropper effect for the light of a receding source"
    √((1+vs/c)/(1-vs/c) * λo

    3. The attempt at a solution
    I tried using 0.3c for the speed of the source, and 650 for λo. (I know the answer is 800 nm); I then tried using 0.1c instead. I got (almost) the right answer though when I plug in 0.2c... the wavelength came up as 796 nm.

    Don't know what else to use!
  2. jcsd
  3. Mar 29, 2012 #2
  4. Mar 29, 2012 #3

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Remember that the two cited speeds (and they should have been! :rolleyes: ) are both with reference to Jupiter. It's not that you are observing the speeder going by you at 0.2c. Each has his own speedometer and those readings are 0.1c and 0.3c.

    You need two formulas. One to give you the true relative speed between the two rocket ships and the other to compute the Doppler shift relativistically.

    BTW I got 801.2368685 nm.
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