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I was under the impression that neon lights work by fluorescence and therefore work by emitting a certain wavelength of light. neon produces an orange color, hydrogen a red color. Supposedly, they use carbon dioxide for white neon lights. But isn't white light a mixture of multiple wavelengths. What am I missing here?

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I believe the "neon" lamps that you are referring to (tubes that light up in different colors but look transparent when they are off) are actually low -pressure gas discharge tubes. Electricity is applied across the gas, causing the gas to emit a discrete discharge spectrum with wavelengths which are characteristic of the gas.

Only specific wavelengths are emitted, but several wavelengths may be present depending on the electronic structure of the gas molecule involved. Neon, for example, has many wavelengths in the red range, hydrogen has 3 strong lines and a few more weak visible-range lines that combined produce a purplish glow. Helium has a set of widely-spaced visible lines that combine to give a salmon-pink glow. Carbon dioxide has a combination of lines which our eyes and brains detect as "white."

Fluorescence is a different process in which a high energy photon (usually in the violet or ultraviolet range) is absorbed by a solid or liquid which consequently emits a lower energy photon. For fluorescent bulbs, the coating on the inside of the tube determines whether the bulb is called "cool white" or "daylight" or "warm." The gas is something (often mercury vapor) which emits a strong UV line via the discharge process.

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