• Question: What is the difference between a sodium vapour lamp, Which gives out a yellowish colour illumination, and a mercury vapour lamp that gives out a white colour illumination?

    Asked by ladysovereignezza to David, Luna, Mark, Melanie, Probash on 21 Mar 2011 in Categories: .
    • Photo: David Pyle

      David Pyle answered on 20 Mar 2011:

      When an electrical current is passed through an ionised gas, the energy excites the valence electrons in the ions, which then release energy at characteristic wavelengths. Mercury vapour emits light at a number of wavelengths in the ultra-violet and visible parts of the spectrum. The UV component is usually filtered out, and the mixture of visible wavelengths appears white. Sodium vapour emits at only one wavelength, which is in the yellow part of the spectrum (near 590 nanometres).

    • Photo: Melanie Stefan

      Melanie Stefan answered on 21 Mar 2011:

      Every element has a specific spectrum of light that it emits (which is why we can figure out the chemical composition of distant stars just by looking at them). Lamps containing mercury have a whiter light, which makes it easier to recognise colours (compared to the light that sodium lamps give).

    • Photo: Luna Munoz

      Luna Munoz answered on 21 Mar 2011:

      Sorry, I don’t know. However, I would say that, based on your question, the difference is the colour they emit 😉

    • Photo: Probash Chowdhury

      Probash Chowdhury answered on 21 Mar 2011:

      I’m not sure of the specifics, but it’s to do with the wavelength of the light that is emmited when energy is passed through the vapour. Sodium glows in a shorter and narrower band of light of the visible spectrum (hence why it is yellow and a pure colour) where as mercury glows at many overlapping wavelengths in the visible spectrum – and as you know when you mix colours they make white.

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