Absorption Spectra Demonstration

Hi Folks,

I'm looking for help setting up a demonstration that used to be part of the old higher physics course. I have very simple notes but looking for more detail if anyone can help.

The demonstration goes as follows:

  • White light is shone through a flame onto a white screen
  • White light is shone through a flame with a 'sodium wick' or 'sodium pencil' burning in it
  • Sodium lamp is shone through the flame onto a white screen
  • Sodium lamp is shone through the flame with a 'sodium wick' or 'sodium pencil' burning in it
I think what's supposed to happen is that no shadow appears on the white screen when either white or the yellow light is passed through the flame, however, this shadow appears when yellow light is passed through the flame produced by the burning sodium wick.

In this scenario, is a burning wick just a wooden taper soaked in a sodium salt? I can't see any 'sodium pencils' in our store and I can't find any information about them online.

What would act as a good source of white light? I have an old projector which I'm thinking may be helpful, going to give it a try later on.

Thanks for any help :)
 
Hi Folks,

I'm looking for help setting up a demonstration that used to be part of the old higher physics course. I have very simple notes but looking for more detail if anyone can help.

The demonstration goes as follows:

  • White light is shone through a flame onto a white screen
  • White light is shone through a flame with a 'sodium wick' or 'sodium pencil' burning in it
  • Sodium lamp is shone through the flame onto a white screen
  • Sodium lamp is shone through the flame with a 'sodium wick' or 'sodium pencil' burning in it
I think what's supposed to happen is that no shadow appears on the white screen when either white or the yellow light is passed through the flame, however, this shadow appears when yellow light is passed through the flame produced by the burning sodium wick.

In this scenario, is a burning wick just a wooden taper soaked in a sodium salt? I can't see any 'sodium pencils' in our store and I can't find any information about them online.

What would act as a good source of white light? I have an old projector which I'm thinking may be helpful, going to give it a try later on.

Thanks for any help :)
There's a video of the Sodium absorption on my FB page. MrQ The Science Guy.
 
Thanks for the video, it gave me some good ideas but unfortunately it seems our sodium lamp isn't working so we won't even be able to do the demonstration just now.

Some useful links for anyone that might come across this in future:




 
I just bought a low pressure sodium lamp on ebay. They have another one, not quite as flexible as mine but you just need to add a retort stand.
Turns out our lamp is probably fine, it's our lamp holder that is dodgy, it keeps blowing the fuse. I changed the fuse for a higher rating and it blew the fuse again and now the switch at the socket is stuck in the 'on' position.
 

Nick Mitchener

COMMITTEE
Turns out our lamp is probably fine, it's our lamp holder that is dodgy, it keeps blowing the fuse. I changed the fuse for a higher rating and it blew the fuse again and now the switch at the socket is stuck in the 'on' position.

It needs a ballast too, perhaps the problem is in that area? The lamp holder is just BC or ES so easily changed.
 
It needs a ballast too, perhaps the problem is in that area? The lamp holder is just BC or ES so easily changed.
I thought it was missing something but the teacher insisted it was fine to plug into the mains. We have another lamp holder (which also doesn't work) and this has a controller and transformer built in.

Do you have an idea of what a ballast for the lamp holder would look like?
 

Nick Mitchener

COMMITTEE
Just wondering as I might already have one lying around without knowing that's what it's used for.

An anonymous looking box with connections on the end usually. Also needs a capacitor. The boxes usually have a circuit diagram on top for connections or you can get them all enclosed as a power supply with an input from a 3 pin socket and an output to the lampholder.
 
An anonymous looking box with connections on the end usually. Also needs a capacitor. The boxes usually have a circuit diagram on top for connections or you can get them all enclosed as a power supply with an input from a 3 pin socket and an output to the lampholder.
Don't think we have one unfortunately. Will need to convince somebody to get funding for a new lamp holder and control transformer. Thanks for your help Nick :)
 
That will cost you around £200-300
Snap that one up while it is still there!
Yeah very expensive, although I'm not sure the polarimeter would be useful for the absorption spectra demonstration. We do have a functional sodium lamp (I think), I'm sure it's just a holder and PSU that we need. The teachers are quite specific about what they want to show and barely move outside the curriculum in physics, so unless the polarimeter can show absorption spectra, it would be wasted here. Not to mention the school won't allow me to purchase equipment unless it's from a reputable education supplier.
 
We do this experiment. We used to use sodium pencils but they appear to no longer be available. Instead, we use some corrugated cardboard soaked in saturated NaCl solution. We then wrap the cardboard round the collar of a bunsen burner and light the bunsen. It's a bit messy, but it works. For a white light, we have a 12v halogen lamp, it's an old Griffin and George lamp, but again, it does the job.
 

Nick Mitchener

COMMITTEE
Yeah very expensive, although I'm not sure the polarimeter would be useful for the absorption spectra demonstration. We do have a functional sodium lamp (I think), I'm sure it's just a holder and PSU that we need. The teachers are quite specific about what they want to show and barely move outside the curriculum in physics, so unless the polarimeter can show absorption spectra, it would be wasted here. Not to mention the school won't allow me to purchase equipment unless it's from a reputable education supplier.
It's the lamp you want. not the polarimeter, you could flog that afterwards.
 
An anonymous looking box with connections on the end usually. Also needs a capacitor. The boxes usually have a circuit diagram on top for connections or you can get them all enclosed as a power supply with an input from a 3 pin socket and an output to the lampholder.
the rh one is an old griffin one with round plugs for the tube part. if you have the tubes you must have the housing and transformer unless someone threw them away.

1678444612812.png
 

Nick Mitchener

COMMITTEE
the rh one is an old griffin one with round plugs for the tube part. if you have the tubes you must have the housing and transformer unless someone threw them away.

View attachment 11123
My one is a Bellingham & Stanley, that has a small 3 pin outlet for the lamp housing
I've just been using it with an 1880s spectroscope. I'm hoping to be able to project with a large enough diameter spectroscope, but might need a slide projector for the lamp and burn sodium for the absorprtion spectrum to be bright enough.
 
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