... ions and electrons carries the electric current between the tube's electrodes. The photo (above) shows a gas discharge sign. This sign incorporates a neon discharge tube (the orange word "Physics") and mercury discharge tubes (the blue word "Experience" and the outer frame). The sculpture at the bottom of the sign represents the electric and magnetic fields of light. The white and yellow sine waves in the sculpture are actually fluorescent lights. These fluorescent lights are mercury discharge tubes with special coatings on their inner walls. The ultraviolet light emitted by the mercury discharge inside a tube is absorbed by the coating, which subsequently emits light of a different colour (and with a lower photon energy). Depending on the exact material of the coating, a whole range of colours can be obtained. So why do these gas discharges emit light? As an alternative to being removed by an energetic collision, an electron ...

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