[mike]

Someone out there may be able to explain this for me. This is a weaker area of my knowledge.

I just did an "experiment" where I did the standard flame test for sodium (NaCl solution on wire in bunsen burner) and get the yellow flame. This part is fine, I then did the same test in front of a large sodium lamp and the flame goes dark/black. Now it looks cool, but I am curious as to what occurs here.

Are the sodium atoms emitting and absorbing energy at teh same time so as to cancel the process out?

I thought maybe that the majority of the substance in the "flame" may be gaseous sodium and is simply absorbing the light from the sodium lamp making a dark spot apper in front of the lamp.

[Borek]

I suppose it is only effect of 'relative brightness' seen by the eye and analyzed by brain, not a real physical process. Flame color is identical with the bright lamp in the background, so it is no longer the brightest spot.

Does the flame look dark when the sodium lamp is on the side, so that it is not blinding you and only the flame is lighted (you don't see any reflections that could be blinding)? If so, there is some physics to it, If not, it is only physiology

[mike]

Yes you could be right. I have a feeling there is more to it though. I suspect it is a similar concept to the dark lines visible in the solar spectrum. If you were to shine a white light sample through gaseous sodium it should absorb the "sodium lines". So if you shone a sodium light (consisting of just the sodium lines) through the same sample would it not be absorbed with no transmitance?

Mike

[Borek]

Obvious No idea why I missed this one.

Still, the flame only 'seems' dark due to the backround brightness.

On the second thought... you may be able to spot that the flame observed from side gets brighter when the lamp goes on - that will be super experiment!

[GCT]

Yes you could be right. I have a feeling there is more to it though. I suspect it is a similar concept to the dark lines visible in the solar spectrum. If you were to shine a white light sample through gaseous sodium it should absorb the "sodium lines". So if you shone a sodium light (consisting of just the sodium lines) through the same sample would it not be absorbed with no transmitance?

Mike

not quite sure, but you probably just have absorption going on, the emissions are essentially being absorped.  You've got a source Na+ lamp (at times used in AAS) which emits certain discreet wavelengths (with a natural line width) and a sample of monoatomic Na+ gas produced by the flame; you've essentially got a setup for obtaining an absorbance spectrum of a sample.  

[mike]

ANother interesting point is that the sodium lamp is an old street lamp, I don't know if you guys have sodium street lamps where you are. I think it is cool looking at different lights (and the sun) with a hand held spectrometer (which we make over here for one of our labs). The fluoro lights are great as you can see mercury emmissions.

[Borek]

I believe you can try to make your own "spectrometer" using CD as a difraction grating. Could be an interesting experiment.

[mike]

Yes I have read about these spectrometers that use a cd as a diffraction grating. I would love to make one of these one day.

The ones I make at work are a cardboard box with a diffration grating that doubles as an eyepiece and a slit at the opposite end. The diffration grating not only splits the light but reflects it onto a rough scale in nm.

Do you have a reference or URL for the CD spectrometer?

[Borek]

Do you have a reference or URL for the CD spectrometer?

No. I think I have seen the idea at sciencemadness few months ago, but it was mentioned only.