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So I've conducted an experiment to find the four visible hydrogen emission spectrum lines in the Balmer series in a laboratory. I don't have any background in quantum physics.

When I looked through the eyepiece, I saw the red light, the pale blue light, and the purple light as shown in the picture below: enter image description here

I've asked my lab instructor why couldn't I see the blue light and he said it is well-known that sometimes not all the spectrum is shown. He told me to look it up on the internet, since I didn't take the quantum physics course yet.

Can someone explain to me this phenomenon? Or at least refer me to an article which discusses this issue? The experiment revolved around Balmer series only. (It was my first spectroscopy experiment)

The setup looked like this:

It looked like this: enter image description here

Edit: These are the wavelengths I've found: enter image description here

So I think the missing wavelength is actually the $410 nm$.

What can be the reason for this?

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  • $\begingroup$ You haven't told us anything about either the instrument you used (aside from it's having an eyepiece) or the source (I'd guess a gas discharge tube, but you haven't said). How are we suppose to help? $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Nov 18, 2015 at 16:47
  • $\begingroup$ I must admit I can't think of a well known reason why the 434nm should be missing. The short wavelength lines are generally a lot fainter than the red and cyan lines, but that would apply to the violet line at 410nm as well. $\endgroup$
    – John Rennie
    Commented Nov 18, 2015 at 16:54
  • $\begingroup$ I've got two decent spectroscopes, one prism based, one diffraction grating based, and can confirm that the $410.1\:\mathrm{nm}$ line is hard to see. I use a typical gas discharge tube to produce the spectrum. It probably depends on quite a few factors like intensity of source, instrument, how well you block stray light and the state of your eyes whether you will see it or not. Maybe try again after allowing your eyes to get used to the dark, as you would looking through a telescope? $\endgroup$
    – Gert
    Commented Nov 18, 2015 at 16:54
  • $\begingroup$ Both me and my partner were in a dark room for half an hour, and we couldn't find that single line. Our instructor couldn't find it either. $\endgroup$
    – Alexs68
    Commented Nov 18, 2015 at 16:58
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    $\begingroup$ Can we establish what the wavelength dependence of the spectroscope plus grating is? In general, unless using specialised glasses and gratings one does expect a reduced efficiency as one heads towards the UV. $\endgroup$
    – ProfRob
    Commented Nov 18, 2015 at 22:32

2 Answers 2

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I was looking through some NIST atomic data for the Balmer Series from here: http://www.nist.gov/srd/upload/jpcrd382009565p.pdf

It lists the spontaneous emission rates for the Balmer series as follows:

  • $\lambda = 656 \text{ nm}$, $A_{32} = 4.41\text{e+}7\text{ s}^{-1}$
  • $\lambda = 486 \text{ nm}$, $A_{42} = 8.42\text{e+}6\text{ s}^{-1}$
  • $\lambda = 434 \text{ nm}$, $A_{52} = 2.53\text{e+}6\text{ s}^{-1}$
  • $\lambda = 410 \text{ nm}$, $A_{62} = 9.73\text{e+}5\text{ s}^{-1}$
  • $\lambda = 397 \text{ nm}$, $A_{72} = 4.38\text{e+}5\text{ s}^{-1}$

So, the lines should certainly get dimmer as you move toward the UV, but I don't see any reason from a quantum mechanics perspective that the blue line at $\lambda = 434 \text{ nm}$ should be dimmer than the line at $\lambda = 410\text{ nm}$

Based you edit above, you seem to be missing the 410 line. It might not be seen because 1. the spontaneous emission rate is lowered and 2. your eyes are not as sensitive in this part of the EM spectrum. 3. As dmckee pointed out in your comments section, being a higher excited state, it will also be less populated that lower lying levels, meaning the total spontaneous emission from this state will be even further decreased.

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  • $\begingroup$ I stand corrected, the missing wavelength was $410 nm$ So the explanation is that the purple line is dimmer then the other lines? Isn't there a better answer? $\endgroup$
    – Alexs68
    Commented Nov 18, 2015 at 17:56
  • $\begingroup$ @Alexs68 I edited my answer correspondingly. Its probably do to both the lowered spontaneous emission rate and your eyes not being sensitive when you get to low wavelengths. $\endgroup$
    – tmwilson26
    Commented Nov 18, 2015 at 17:57
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Why is the blue line in the Balmer series sometimes not visible?

The human eye has difficulties in distinguishing dark blue lines on a black background.

You can use "Microsoft Word" to draw a black rectangle and a few dark blue lines of different thicknesses on the rectangle. The thinner the dark blue line the less visible it is. Even the thick dark blue lines are not quite visible.

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