Uploaded January 2025 | Updated September 2026, 3 weeks ago
This is more of a physics experiment to start the year off and demonstrates spectral-line broadening with increasing vapour pressure in the sodium discharge. The video is too sort to delve into all the details but as an introduction to the slightly unintuitive concept of the dark space in the sodium-double line becoming broader. There are a number of processes responsible for line-broadening with the simplest being Doppler. Where the molecules are moving faster their emissions become red or blue shifted depending on how they move to the observer. This can’t explain the dark space as this would disappear rather than broaden. So, the process of resonant self-absorption extends with the pressure increase and is removing all of the original wavelengths visible emissions in the low-pressure lamp. This resonance absorption is very prominent with sodium vapour and the mechanism in ‘black flame experiments’.
My experiment uses two industrial lamps. The first is a 35w SOX LPS and the second is a 600w SON/T HPS. Both are running on electronic ballasts so no flicker is observed on the video. It is also interesting to note there are starter noble gasses in side both lamps and the addition of mercury in the HPS lamp, but sodium is the dominant emitter by far having the lowest work function causing the lamp voltage to drop to a point the other gasses don’t ionise. The mercury does play an important part in the HPS lamp line broadening but not with the addition of mercury transitions.
This is more of a physics experiment to start the year off and demonstrates spectral-line broadening with increasing vapour pressure in the sodium discharge. The video is too sort to delve into all the details but as an introduction to the slightly unintuitive concept of the dark space in the sodium-double line becoming broader. There are a number of processes responsible for line-broadening with the simplest being Doppler. Where the molecules are moving faster their emissions become red or blue shifted depending on how they move to the observer. This can’t explain the dark space as this would disappear rather than broaden. So, the process of resonant self-absorption extends with the pressure increase and is removing all of the original wavelengths visible emissions in the low-pressure lamp. This resonance absorption is very prominent with sodium vapour and the mechanism in ‘black flame experiments’.
My experiment uses two industrial lamps. The first is a 35w SOX LPS and the second is a 600w SON/T HPS. Both are running on electronic ballasts so no flicker is observed on the video. It is also interesting to note there are starter noble gasses in side both lamps and the addition of mercury in the HPS lamp, but sodium is the dominant emitter by far having the lowest work function causing the lamp voltage to drop to a point the other gasses don’t ionise. The mercury does play an important part in the HPS lamp line broadening but not with the addition of mercury transitions.
