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Investigation about the emission spectrum of Mercury

  1. Jul 15, 2017 #1
    1. The problem statement, all variables and given/known data

    For our investigation, we used a diffraction grating spectrometer to observe the emission spectrum of mercury.
    We conducted the experiment in a pitch black room to avoid any unwanted light pollution, etc.
    Whilst conducting the experiment we observed a number of 'faint' lines surrounding each of the brightest emission lines. For example, several faint blue spectral lines were observed surrounding the blue spectral line with the greatest luminosity. You can see these here: http://imgur.com/a/6KsOh

    I was wondering if what we observed was perfectly normally, and if there is a name for it as well as a good explanation to include in my report.

    2. Relevant equations

    3. The attempt at a solution
    I have noticed that what we observed are somewhat similar to the following image of the Mercury emission spectra: https://i.stack.imgur.com/onjhd.png
    There appears to be some sort of light with a lower luminosity in-between each of the bright spectral lines, especially the red. However, I don't know the meaning of this.
    My Physics teacher is ill and has been on leave, and I have been unable to make contact with him. I asked the other physics teacher at my school and he was unable to explain our observations.

    Any help would be much appreciated :) Thanks
  2. jcsd
  3. Jul 16, 2017 #2


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    My understanding is that a diffraction grating is equivalent to a uniformly illuminated antenna array.
    Such an array has a succession of sidelobes each side of the main beam and these gradually get less as we move off axis. The Wiki reference below gives sidelobe levels as -13dB (intensity 0.05) for the first and-17dB (intensity 0.02) for the second.
    I should mention that although this is the principle of the effect, for the optical case we are working very close to the array, in the Fresnel Region, so that these numbers will not necessarily be correct.
  4. Jul 16, 2017 #3
    Thanks for the reply :) This seems to support what we observed in the investigation. I just wanted to clarify whether these side lobes are what can be seen circled in the following image: http://imgur.com/a/BzJWL ?
  5. Jul 16, 2017 #4


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    My picture here is not very sharp and I cannot see the lines in a group of three (or five etc) as I was expecting.
  6. Jul 16, 2017 #5

    Charles Link

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    It's difficult to assess without additional experimentation. If you used a lens to focus onto the entrance slit to illuminate much of the diffraction grating (giving the beam a healthy f #), or even illuminate more than a very small region on the diffraction grating, there are enough lines on the grating that you would not get secondary maxima that far away from the spectral line that are that prominent. ## \\ ## If you have no ghost reflections occuring in your spectrometer (e.g. reflections off of a detector window that cycle through again), my guess is that these are additional and much dimmer lines in the Hg spectrum that you are also observing. ## \\ ## Note: If the diffraction grating is used in the proper manner as a many-lined grating, where you use at least approximately100 lines on the grating (the beam incident on the grating should be at least 1/4" across approximately), any significant secondary maxima will be so close to the primary maximum that they basically become part of the primary maximum. ## \\ ## Editing...I don't have a copy handy of a Handbook of Spectroscopy, but as I recall, mercury does have quite a number of other emission lines besides the very prominent ones. (That normally is the case with these larger atoms=e.g. the emission spectrum of iron consists of many many lines.) Also, an additional bit of googling showed a couple of spectral images that were taken that show this is the case.
    Last edited: Jul 16, 2017
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