# B Obtaining spectrum of Sodium from Sodium Vapour Lamp

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1. Jul 30, 2017

### Wrichik Basu

I learnt that two lines in the emission spectrum of Sodium, namely $D_1$ and $D_2$, are on the visible region of the spectrum.

Can I obtain these lines from a Sodium vapour lamp? I want to use a prism to separate out the individual wavelengths, a no then use a photographic film to obtain the spectrum on a screen. In this case, will I be able to see the two lines?

Please note that I want to conduct this experiment in my school laboratory. So, I don't have very good instruments other than a simple student spectrometer, which is used to determine the angle of prism and it's minimum deviation. It is for the use of this spectrometer that the school lab has a Sodium vapour lamp.

2. Jul 30, 2017

### pixel

The sodium D lines are about 0.6 nm apart. Do you know the resolution of your spectrometer and whether it is capable of resolving the two lines?

Also, I assume you are using a low pressure sodium lamp. In a high pressure lamp, the lines are broadened and may not be resolvable.

3. Jul 30, 2017

### sophiecentaur

Film involves a lot of work and, these days, the camera on a phone is capable of giving you a very good result. If you can see the lines through the eyepiece of the spectrometer, you will probably be able to photograph what you see. You may have to fool around with focussing the eyepiece to get a result but it is certainly worth a try.
As pixel says, you need to check that the lines are actually resolved in the first place.

4. Jul 30, 2017

### Wrichik Basu

You are slightly mistaken. I want to use a prism rather than a spectrometer, because this spectrometer is of low quality and a very very simple one. I don't think I can use this one here.

5. Jul 30, 2017

### Wrichik Basu

How will I know that the lines I am seeing are the spectral lines? Do you mean to say that if I see lines after the light passes through a prism on the spectrometer, they are the D lines?

6. Jul 30, 2017

### sophiecentaur

Well then, use a lens and examine the spectrum that the prism gives you. Your alternative prism system needs more that just a prism. You will need a well collimated beam and a very thin slit. You say that your spectrometer is perhaps not good. Have you set it up correctly yet? Could se have a photo of it, perhaps? Is it more of a Spectroscope than a Spectrometer - a small cylindrical, hand held device?
You need to look at Images of Spectrometers. Google Images "spectrometer" to see dozens of suitable pictures and links to give you ideas.

7. Jul 30, 2017

### Wrichik Basu

I've posted a picture above. Posting it again:

Although this is not the actual photo of the instrument, the one in my school is of this type.

8. Jul 30, 2017

### sophiecentaur

Sorry - I missed the picture lat time round.
So what is wrong with your school model? It may have been mis-used and set up wrongly. I really question whether you will be able to do better. I do remember seeing Sodium D lines when I was at school, on an ancient brass job. The Physics teacher knew what he was doing, though (we had no idea!!) and had obviously set it up. Check the slit and the focussing at both ends. I haven't looked but I would be surprised if you cannot find "setting up a spectrometer" somewhere with Google. Imo, that's the best way to go initially. You could well be pleasantly surprised. AND - you wouldn't;t have to actually make anything.
PS What is the prism like that's in the spectrometer already?

9. Jul 30, 2017

### sophiecentaur

If you use a proper source then you should only see the Sodium lines. The Yellow ones are the ones you want. This should be done in the dark - of course.

10. Jul 30, 2017

### Wrichik Basu

The teachers at my school have know idea about this experiment. I'm trying to design it myself. The spectrometer is just kept for show. But it works. The prism is an equilateral glass one. Could you tell me a bit more about the procedure of the lens and the resolution things....... I know how to set the instrument, but tell me a bit more clearly about the resolution you were taking of.

11. Jul 30, 2017

### sophiecentaur

You will have a hard job doing better than the instrument you already have - seriously.
I suggested that you search with Google for some instructions. I think you are as capable as I am of doing that and the exercise would do you good. You need to become a bit of a minor 'expert' about this so you have to do a fair bit of reading round and teaching yourself to Search for Information. You should look at a good selection of Google hits and you may have to look way down the list before you find something suitable.
"Collimating a Spectrometer" could be another suitable search term.