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Homework Help: The Doppler Effect of light

  1. Sep 11, 2012 #1
    1. The problem statement, all variables and given/known data

    The light from galaxy NGC 221 consists of a recognizable spectrum of wavelengths. However, all are shifted to the shorter wavelength end of the spectrum. In Particular, the calcium "line" ordinarily observed at 396.85nm is observed at 396.58nm. (a) is this galaxy moving towards or away from the earth? (b) at what speed?

    2. Relevant equations


    3. The attempt at a solution
    I think that the galaxy is moving toward the earth, but I'm not sure how to get what speed it's going at. I tried totake the ration of 396.58/396.85 to plug in so the frequencies would cancel out, but I'm getting a very small negative number. This doesn't seem right. Any help would be appreciated!
  2. jcsd
  3. Sep 11, 2012 #2


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    Homework Helper

    Use the formula
    fobs = fsourcesqrt[(c-v)/c+v)]
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