One of the things about city life is the orange glow in the night sky from the large number of street lights. In Nottingham I could never see the Milky Way because of the light pollution. In Wigton it is possible to pick it out. The orange street lights are sodium. The metal is vaporised and placed in a glass bulb between two electrodes. A beam of electrons then courses through the vapour. When an electron hits a sodium atom, it knocks one of the electrons in that atom up several energy levels. As that excited electron falls back down, it emits a photon of light. The frequency (colour) of the photons is fixed by the size of the energy level jump. As these are unique to a particular element, sodium has photons of a different frequency to the mercury street lamps. In sodium this adds up to an orange glow.
