The photo (above) shows a gas discharge sign designed by Sam Sampere of Syracuse University. This neon 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 neon light of a different color (and with a lower photon energy). Depending on the exact material of the coating, a whole range of colors can be obtained.
So why do these gas discharges emit light? As an alternative to being removed by an energetic collision, an electron on an atom can be excited. One speaks of the electron as having been promoted to an orbital of higher energy. When the electron eases back down to its original orbital, a particle of light (a photon) carries away the energy of excitation and the discharge tube glows! A photon's energy (its wavelength or color) depends on the energy difference between orbitals. A given atom can emit photons at many energies corresponding to its different pairs of orbitals. This series of photon energies the emission lines to a spectroscopist is unique to a particular atom. As can be seen in the sign, the mercury discharge tubes have a very different hue than the neon discharge tube does. The inert gas helium was actually discovered this way, and observations of sunlight revealed a series of photon energies that had never before been seen in discharges on the earth.