Fluorescent light bulbs are a type of light bulb that uses electricity to excite mercury vapor. When the gas reaches a certain energy level, it begins to give off photons at certain wavelengths that cause the lamp to produce visible light. Compared to traditional incandescent lamps, fluorescent light bulbs are more costly to purchase, but use electricity much more efficiently. They also last much longer, but are complicated to dispose of properly, and the light they produce does not lend itself to most color photography as well as incandescent light does.
Perhaps surprisingly, fluorescent bulbs have a history that is almost as long as that of incandescent light bulbs. In fact, even Thomas Edison, the inventor of the incandescent lamp, patented an early fluorescent lamp. However, fluorescent light bulbs as we know them today were not on the market until the late 1930s, decades after the incandescent bulb was already in wide use.
The chemical and electrical principles by which fluorescent bulbs operate are quite complex, but the general idea is simple enough to be summed up briefly. Inside the fluorescent lamp is a mixture of gases at very low pressure, which includes mercury vapor. When an electron collides with an atom of the gas, the atom is temporarily raised to a higher energy state.
This new energy state is unstable, though, and as the atom goes back to its normal state, it emits a high-energy photon. This photon collides with an atom in the lamp's fluorescent inner coating, causing a similar reaction, but this time the photon has a lower energy, and can be seen by human eyes. Many such interactions happening simultaneously cause the lamp to give off a large amount of light.
Fluorescent light bulbs eventually "burn out" when the mercury is absorbed by the inner parts of the lamp, and when other chemical balances inside the lamp are lost. During their long life, though, they use much less energy than incandescent lamps do to produce the same amount of light. This efficiency has led to interest in fluorescent bulbs as replacements for older types. In recent years, this replacement -- in the form of compact fluorescent light bulbs -- has seen an increasing degree of popularity.
Compact fluorescent lamps (CFLs) are designed to mimic the light produced by incandescent lamps, and are roughly the same size. They cost much more at the outset than traditional bulbs, but are estimated to save about $30 U.S. Dollars in electricity costs over the life of the bulb. The modern CFL was invented in the 1970s, but has only been produced on a large scale since the 2000s.