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Homework Help: Doppler effect star and earth question

  1. Oct 5, 2006 #1
    Hello I have a question that is giving me trouble:

    Light of wavelength 659.7 nm is emitted by a star. The wavelength of this light as measured on Earth is 661.1 nm.

    How fast is the star moving with respect to Earth? km/s

    Is it moving toward Earth or away from it?

    Here's where I'm at. I have reasoned that since the wavelength is increasing that the star is moving away. My main problem is determining the frequency of the light. I assume that I can use the formula:

    velocity = wavelength * frequency

    however the frequency I get just seems wrong. The units are throwing me off too, any help would be greatly appreciated.
  2. jcsd
  3. Oct 5, 2006 #2


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    Staff: Mentor

    What is the equation for the change in frequency of a wave based on its speed and the speed of the emitter? And keep in mind that the speed of light is about 300*10^6m/s (check this before using it, though), so that gives you a way to convert between frequency and wavelength.

    EDIT -- Fixed typo in speed of light. It's 300*10^6, not *10^8. Sorry.
    Last edited: Oct 5, 2006
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