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Homework Help: Spectral Lines

  1. Feb 10, 2014 #1
    1. The problem statement, all variables and given/known data

    In a question I was asked, assuming a spectrometer reading of Hydrogen produced two strong spectral lines at 656.3nm and 410.1nm. And also assuming the diffraction grating had 500 lines/mm

    What is the highest order of spectrum which can be fully observed , i.e value of m.

    and assuming the physicist begins with the spectrometer telescope eyepiece at a deviation angle of zero and rotates it through 90 degrees, determine the sequence of line colours that she sees?

    2. Relevant equations


    3. The attempt at a solution

    I worked out that the highest value of m for the 656.3nm line is 3 and for the 410.1nm line it's four but because it says fully observed I said three because the two lines can be observed fully at this order of spectrum.

    For the second part I wrote out multiples of the wavelengths, I got

    410.1, 820.2, 1230.3, 1640.4

    656.3, 1312.6, 1968.9

    So if I call the 410.1nm one violet, and the 656.3nm one red. Are the colours she sees

    Violet, red, violet, violet, red, violet, red?
  2. jcsd
  3. Feb 10, 2014 #2


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    Homework Helper

    The result is correct, but the physicist observes angles. It does not change the order as the sine function is monotonously increases with your multiples of wavelengths.

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