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Principles Of Operation Of Fluorescent Tubes
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The main principle of how a fluorescent tube works is based around the inelastic scattering of electrons.
A cathode electrode (see image on left, typically made from coiled tungsten that is coated with a mixture of barium, strontium and calcium oxides) emits an incidental electron which then collides with an atom in the gas in the tube (such as mercury, argon, or even krypton).
This collision generates kinetic energy which prompts an electron in the gas atom to jump up to a higher energy level in order to absorb some, or all, of this energy (BAD JOKE: An excited mercury atom goes into a bar and says to the barman, “I think I just bumped into an electron!”. The barman says, “Are you sure?”. To which the atom replies, “Yes, I’m positive!”).
Because the higher energy state is unstable, the atom will emit an ultraviolet photon as it’s electron reverts to a more stable energy level and because the photon emitted is usually in the ultraviolet spectrum, it isn’t visible to the human eye and needs to be converted into visible light.
To accomplish this the inner surface of the fluorescent tube is coated with phosphor. The photons are absorbed by the electrons in the phosphors atoms and a similar energy jump, then drop, emits another photon. This new photon has a lower energy than the one that caused it to be created and any difference in energy is emitted as heat.
The chemicals in the phosphor are specially chosen to emit a photon at a wavelength that is visible to the human eye. The other job the phosphor performs, along with the glass of the tube, is to stop any of the harmful UV light escaping.