• Question: Why when we see electrons passing through the air do they appear to be purple? Also, where does the charge in lightning come from?

    Asked by to Thomas, Saiful, Piers, Olivia, Meggi, Laurence on 13 Jan 2017. This question was also asked by .
    • Photo: Piers Barnes

      Piers Barnes answered on 13 Jan 2017:


      When electrical current passes through air light is often emitted from an ‘arc’ (in lightning for example) or at low air pressures there is a slightly purple glow. This occurs because the molecules in air do not conduct electricity well themselves because they are neutral and not connected to each other. In order for electrical current to pass through air there must be a sufficiently high electric field in the gas for some of the electrons in the outer shell of the gas molecules to be stripped off, creating charged ions. These ions and electrons are then accelerated by the electric field and bump into each other with enough energy to create more ions. The ions often bump into electrons which can then fill the empty shells that had been vacated. When this happens photons can be released with a colour that corresponds to the energy lost by the electron as it falls back into its empty shell.

      A somewhat similar light emission process occurs in lighting. The charge build-up is thought to come from electrons being stripped from the surface of ice particles bumping into falling supercooled water droplets or hail as the ice is swept up within the updrafts in cold interior of cumulonimbus clouds. Rain or hail can bring the negative charge to the ground leaving the clouds positively charged compared to the ground or nearby clouds. This creates sufficient electric field strength to generate an arc (where the gas gets ionized) a lot of energy is released in the arc which heats up the ions so they emit light at many energies resulting in the white emission from lightning.

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