The ultraviolet light emitted by the mercury discharge inside a tube is absorbed by the coating, which subsequently emits light of a different color (and with a lower photon energy). These fluorescent lights are mercury discharge tubes with special coatings on their inner walls. The white and yellow sine waves in the sculpture are actually fluorescent lights. The sculpture at the bottom of the sign represents the electric and magnetic fields of light. This sign incorporates a neon discharge tube (the orange word "Physics") and mercury discharge tubes (the blue word "Experience" and the outer frame). The photo ( above) shows a gas discharge sign designed by Sam Sampere of Syracuse University. The resulting plasma of charged ions and electrons carries the electric current between the tube's electrodes. An ionized atom has had an electron plucked out of an orbital to make it a "free" particle, and the atom it leaves behind has become a positively charged ion. The voltage must be large enough so that this energy is more than that required to "ionize" the atom. The voltage across a discharge tube will accelerate a free electron up to some maximum kinetic energy. It's much easier to explain why neon isn't inert in a discharge tube than it is to explain why it is inert to chemical reactions. One need apply only a modest electric voltage to electrodes at the ends of a glass tube containing the inert gas and the light begins to glow. But it is pretty easy to build a gas discharge tubesuch as a neon lightwhich reveals that inertness is a relative matter. Neon signs are orange, like the word physics above.īy definition, the atoms of inert gases such as helium, neon or argon never (well, almost never) form stable molecules by chemically bonding with other atoms. GAS DISCHARGE TUBES emit different colors depending on the element contained inside.
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