Light bulbs used to be simple: just runs a bunch of electrical
current through a thin wire until it heats up enough to start glowing. Bare
filament electric lamps were first demonstrated around 1800 by Humphry Davy,
and the glass bulb was added later to keep oxygen away from the wire so it
could glow for a long time without actually burning up.
So the incandescent light bulb is 19th-century technology, and by
now there's now a blinding array of electric lamps - halogen light bulbs, fluorescents,
mercury and sodium vapor lamps,
LEDs, lasers and so on. Each one makes its own clever use of physics
to achieve the life goal of a light bulb: converting electrical current into
visible light. Here's how they work.
Halogen bulbs have the same tungsten metal filament as typical
incandescent light bulbs, but they contain a little bit of a halogen-based gas
in the bulb as well. The chemistry of the halogen gas allows it to capture
stray tungsten atoms that evaporate off the filament and shepherd them back to
where they belong, which both prolongs the life of the filament as well as
keeps the inside of the bulb clean and clear.
Fluorescent bulbs are basically gas-filled tubes with electrodes at
both ends electrical current flows from one electrode to the other, and when
the electrons that makes up the current bump into mercury atom in the gas, the
energy of the collision makes the atoms get "excited" that's the
technical term - and the atoms then emit visible and ultraviolet light.
The white coating on the inside of the glass absorbs the ultraviolet
light and re-emits it as more visible light - this process is called
"fluorescence" and is the namesake of the bulb. Because the coating
stops the UV light, it also keeps the bulbs from give you cancer... unless that's
what you want, in which case you can use a tanning bulb with a different kind
of coating.
Sodium, mercury, and metal-halide vapor lamps, which are commonly
used for lighting streets, warehouses, gymnasiums, and other large areas, are
also tubes that run electrical current through a gas. The gas itself emits
mainly visible light so these bulbs don't need a fluorescent coating.
Finally, LEDs are also like fluorescent light bulbs, except replace
the gas with a tiny crystal of semi conducting gallium, and throw away the bulb
- so not like fluorescent bulbs.
But seriously, the semiconductor has two layers, one of which
provides keyed up electrons, while the other provides a place for the electrons
to go and relax and that is the
technical term. All you need is an electrical current to transport electrons
from the party side to the spa side where they release the energy of their
excitement as light. Voilá a light-emitting diode, perfect for human parties.