Why Are Only Sodium Spectral Lines Visible in Sodium Vapor Lamps?

[bejoynp]

Hi

Why do we have only sodium spectral lines in the sodium vapor lamp despite the very low concentration of sodium present in the lamp.

 

[Borek]

Is there anything else in the lamp that can emit light?

 

[bejoynp]

In a sodium vapor lamp, there is 99% inert gas (Penning mixture of Argon and Neon) and 1 % Sodium, but in the spectrum, we can see only sodium lines (despite of low concentration) and no argon or neon lines.

 

[Borek]

What do you know about the Penning mixture?

 

[bejoynp]

I am sorry, I accidentally mentioned about low pressure sodium vapor lamp. I actually want to know why Xenon or Mercury spectral lines are not present in the spectrum of High Pressure Sodium lamp. The concentration of sodium in the lamp is about 1% or less.

 

[MagnetDave]

There's a few issues, but it really comes down to brightness. Sodium is used because it's emission is concentrated in a narrow visible band, and it's much brighter than usual.

The other lines are impossibly dim in comparison (and there may be some absorption by the bulb itself, as well).

 

[K^2]

bejoynp, I think if you look up first ionization energies of Sodium and Xenon, you might find a simple explanation.