• 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 alysha to Hitesh, Hywel, Mae, Nik, Tiffany on 13 Jun 2012. This question was also asked by crazygirls123.
    • Photo: Tiffany Taylor

      Tiffany Taylor answered on 13 Jun 2012:

      Well the sodium vapour lamp uses sodium to produce light, and a mercury vapour lamp uses mercury to produce light. Different chemicals burn with different colours. Sodium when it burns, produces a yellow colour, and mercury when it burns produces a white colour.

      That’s all I know about that!

    • Photo: Hywel Owen

      Hywel Owen answered on 13 Jun 2012:

      These are the two common types of gas discharge lamps, in which a high voltage across the gas wrenches the electrons from the gas atoms. The electrons continuously recombine with the gas atoms to give characteristic wavelengths of emission. In sodium this is yellow, while in mercury it is ultraviolet (i.e. invisible). You only get just these colours at low pressures, for example the yellow streetlamps in most countries; low pressure sodium lamps are cheap and give a useful light for safety.

      If the gas in the glass chambers is at higher pressures the gas atoms have a range of velocities, which means the wavelengths emitted are spread out, so the light is whiter. When you first turn on a mercury lamp most of the mercury is a liquid, and needs time to heat up, so the colour is first bluish (low pressure), but eventually whiter.

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