104 High Pressure Sodium Lamp

There are many interesting features in the spectrum of light emitted by a high pressure sodium (HPS) lamp.

Most of the blue lines are emission lines from mercury.

The dominant light of the HPS lamp comes from the yellowish sodium D-line. In low pressure sodium lamp, the D-line would have been a narrow discrete line (a doublet actually). But in HPS lamps, this line is broadened so much by high pressure that it appears like a continuous spectrum. This broadening is caused by the emitting atoms suffering collisions during the emitting process. Because the collision time is much shorter than the lifetime of the emission process, the uncertainty in the energy emitted is increased (Heisenberg Uncertainty Principle, ΔE Δt > h/4π).

Notice also the dark absorption line in the pressure broadened sodium D-line. This is due to absorption by the cooler sodium at the outer layers of the lamp. Because the probability of absorption is much higher near the line centre than at the wings, the sodium D-line undergoes a self-reversal near the line centre.




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