Experimental demonstration of an absorption spectrum

[DDesulgon]

Hello,

is there a convincing experiment to visualize absorption spectra? At disposal I have:
- Halogen lamps
- Lenses
- Straight view prism
- transmission grating
- slit, iris, screen
- Sodium vapor lamp and mercury vapor lamp

Unfortunately, I do not have a sodium vapor cell or anything similar. I tried to illuminate a flame colored by common salt with the halogen lamp and project the spectrum behind it with slit, lens and prism, but could not see any black lines.

I read in a book that you can also use the sodium vapor lamp as a vapor cell, so illuminate the sodium vapor lamp with a halogen lamp to get an absorption spectrum. However, I am having trouble understanding this approach. When you illuminate the sodium vapor lamp that is turned on, it is so bright that you see a bright yellow line in the spectrum instead.
Or should the sodium vapor lamp be used in the off state?

Does anyone have an idea how to get an absorption spectrum (maybe even the Fraunhofer lines in the solar spectrum?) with the (simple) means available?

Thanks in advance!

 

[tech99]

Some years ago a teacher asked me to do this demonstration, which I did using a sodium flame having a piece of Sodium Hydroxide in it. From memory, the sodium light was focused on to a little spectroscope, and the white light was also focused on to the flame and then on to the spectroscope. So I could display either the spectrum of the white or yellow sources. By increasing the white intensity above the yellow, you could see the black lines. By reducing it below the yellow, you could see the yellow lines, so it works similarly to the Bunsen Grease Spot Photometer. The two sets of lines were in the same place. I found that an absorption spectrum is best seen by passing the white light a little above the luminous flame, so it is in Sodium Vapour which is hot but not radiating.

 

[DDesulgon]

The concept sounds good. Unfortunately, I don't have a professional spectroscope, only a small handheld one.
Is it also possible to project the spectrum onto a screen using a slit, a lens and a grating?

 

[sophiecentaur]

The concept sounds good. Unfortunately, I don't have a professional spectroscope, only a small handheld one.
Is it also possible to project the spectrum onto a screen using a slit, a lens and a grating?

If you want to prove / demonstrate the existence of absorption then take your hand held spectrograph and look at sunlight through clouds. I have a £30 cheapo spectrograph from eBay and the spectrum is full of very visible vertical stripes around the red / Orange region. The spectrum from a halogen (or even a modern LED) lamp is far more continuous so my stripes have to be due to absorption. I must say, I did suspect the instrument before I checked it out with a good reference.
If you want to use a sodium vapour lamp, why not turn it off after it's reached its running temperature and look at the sun through it? The sodium would take a while to condense back to where it came from. If you mask off any direct sunlight, I'd bet you would get a dark stripe where the sodium lines are. Whether you could actually split them would be another matter. I never tried a Neon lamp but you could probably get some absorption of sunlight through it.

 

[DDesulgon]

Thanks for your ideas!
Of course, I have already tried your first suggestion. I really saw some gossamer lines, but wasn't sure if they were really Fraunhofer lines. Especially since I also saw some horizontal black lines because the slit was set so narrow.
Your second suggestion sounds good, I will definitely give it a try.

 

[tech99]

Further to my description, I used a little hand spectrometer. I think I used a camera to display the results but not sure now. There is not enough light for projection.

 

[sophiecentaur]

I really saw some gossamer lines, but wasn't sure if they were really Fraunhofer lines.

Yes; I really doubted the equipment, as would anyone who only paid £30. But as a check, you wouldn't find most of those lines in a domestic lamp. I'd be really interested to find out if any more kitchen table spectroscopy works convincingly. Grandchildren are often bored impressed by some very easy demos when it's not part of a formal lesson.

 

[sophiecentaur]

if any more kitchen table spectroscopy works convincingly

The spectra of CFLs looks so dodgy and it's really no surprise that the colour of everything you see, under them, is unbelievable weird. Worth looking at, though.