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Hydrogen Spectroscopy

  1. May 6, 2012 #1
    I have asked a question about this before but this is a different question.

    The experiment that my lab partner and I performed used a 360 degree rotating assembly which had the light source (Hydrogen and Sodium) and the telescope at opposite ends while the diffraction grating sat in the middle.

    The trouble is, I did not know that we were supposed to view the "spectroscopy beams" on each side (left and right). This is the confusion, I have angles (I'll post them here tomorrow when I can access my room in daylight as my roommate is sleeping) but I don't know if I measured left and right.

    Is it possible to view the first and second order of Hydrogen on one side only? (think a quarter of a circle and you decrease angle).

    I don't know anything about the machine, I wasn't aware of it at the time but I didn't take into account where the light source was (what angle relative to the whole assembly) and where the scope was...

    So far I am able to get that the wavelength of the red beam which was at 205 degrees, to be 680nm roughly, I couldn't get a real answer without subtracting 360 by 205 degrees, which I don't know if this makes sense. Did I just get lucky? Other groups who also performed the experiment had angles with a range of 4 to 16 or more but it was a small number compared to 155 degrees.

    Can someone shed some light on my meagre mind?
  2. jcsd
  3. May 6, 2012 #2


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    Is it possible to just ignore the others? Of course. Is it still possible to calculate the wave length?

    Well, how much do you know about your setup? Was the incoming light (roughly) orthogonal to the diffraction grating? In that case, the angle of the first and second orders are sufficient. If not, the angle between diffraction grating and light source might be a problem.
  4. May 7, 2012 #3
    I thought that the light source had to be? (Most optimal anyway)

    See I want to use the reason that if you take the angle of 204 degrees, subtract it from 360 you're left with 156 degrees, which is the same as 24 degrees (if you're splitting the circle into two half circles).

    The numbers work out, if I do it that way, but my prof tells me that we were supposed to measure from each side (imagine the light source has a vertical line cutting the circle in half) we were supposed to measure on each side of this line and cut the displacement in half to get our angles.

    When I try this my wavelengths do not make sense, however when I do the computations using sin(360-204) it works out, my red spectral line is 40nm over but that's not bad considering the size. Still it does seem high with the minutes/seconds/hours accuracy. eg. 204+(20/60)+(7/60)+(40/3600) degrees
  5. May 7, 2012 #4


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    Does the 204 have any meaning as individual value? Does 360? Or is it just a random position on your scale?
    Without some sketch, it is hard to guess where your values come from.

    If you measure both sides, you can reduce some experimental errors.
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