Why spectrum of Na vapour spread out when pressure is increased to it?

My teacher told it is bcoz it becomes liquid but i wonder the gas molecules are more loosely packed than liquid molecule then it should have been just opposite.

When in a vapour state, the atoms are spread out. Considering their sizecompared to the volume of the discharge tube, the distances between the atoms can be considered so large that they do not influence each others energy levels. Thus the spectrum that we see is of non interacting atoms and hence consists of discrete lines which correspond to allowed energy levels/transitions.

But in a liquid and more so in a solid the atoms are closer and hence interact. This results in more allowed values of energy which are very close to each other. Hence they appear as a band or more "spread out".

It is from quantum mechanics applied to solids/liq that band theory arises.

What governs the seperation in lines is what is allowed according to band theory.

So when we speak of the spectrum spreading it is something like a line drawn with a pencil compared with one drawn with a brush or drawn with ink which has spread or smudged. It appears thicker and closer actually not lines with bigger gaps as one might think.

And this closeness in the lines of the spectrum is not just because the atoms are physically close like people bunched together with torches in their hands, but because this leads to interaction, which leads to a larger number of closely spaced allowed levels. When light is emitted, the emerging colours (frequencies) are very close and hence there is not much seperation when you view them thru a spectrometer. They thus appear as bands rather than lines and it is then that we say that the spectrum has spread.

hi physicsquest, in this case does band light( as u said) moving from cathode to anode spreads out as (brushed paint immersed fom line sketched by pencil ) on approaching anode?

No. I think the whole process is not clear to you.

The light does not move from anode to cathode in the discharge tube, a current does. This excites the atoms of the gas and these atoms go to a state of higher energy. When they fall back to the lower state, this excess energy is what is emitted as light.

This emitted light is then passed thru a spectrometer which you can simply think of as a prism which disperses the light or splits it into all the colours that are emitted. These colours therefore emerge out of the prism at different angles.These colours are viewed thru an eye piece which can be moved in a horizontal circle so that these colours can be studied individually. The horizontal circle is also marked so that we can find the angle. (Since each colour comes out of the prism at a different angle) In normal white light since all colours are present we see a band of colurs like a rainbow. However when we view the light of sodium vapour thru the eye piece we dont see a band but a set of lines which may look roughly like the first line. After it becomes a liquid it will look more like the second line. (google for "sodium spectrum" , "band spectrum" and you will find plenty of stuff)

| | | |
| | || | | | |

So you see what is a single line earlier has split or spread in the lower line into two lines

In fact when you actually see it these two lines may appear as just one thick line.

So the spread that is being talked about is not in the tube, but in the light emitted by the tube when viewed thru a spectrometer.