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The most important element in a fluorescent lamp is the sealed glass tube. The tube contains a small bit of mercury and a gas, argon, kept under very low pressure. The tube is also coated along the inside of the glass with phosphor powder. The tube has two electrodes, one at each end, which are wired to an electrical circuit. The electrical circuit, is hooked up to an alternating current (AC) supply. .
When you turn the lamp on, the current flows through the electrical circuit to the electrodes. The voltage across the electrodes will make the electrons migrate through the gas from one end of the tube to the other. This energy changes some of the mercury in the tube from a liquid to a gas. As electrons and charged atoms move through the tube, some of them will collide with the gaseous mercury atoms. These collisions excite the atoms, bumping electrons up to higher energy levels. When the electrons return to their original energy level, they release light photons. .
The wavelength of a photon is determined by the electron arrangement in the atom. The electrons in mercury atoms are arranged so that they mostly release light photons in the ultraviolet wavelength range. Our eyes don't see ultraviolet photons, so this sort of light needs to be converted into visible light to illuminate the lamp.
Phosphors are substances that give off light when they are exposed to light. When a photon hits a phosphor atom, one of the phosphor's electrons jumps to a higher energy level and the atom heats up. When the electron falls back to its normal level, it releases energy in the form of another photon. This photon has less energy than the original photon, because some energy was lost as heat. So in a fluorescent lamp, the phosphor gives off white light we can see.
Incandescent light bulbs also emit a lot of ultraviolet light, but they do not convert any of it to visible light. Therefore, a lot of the energy is wasted.
