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 make up the current

bump into mercury atoms 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 bulbs. Because

the coating stops the UV light, it also keeps the bulbs from giving 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 semiconducting gallium, and throw away the bulb - so not like fluorescent bulbs.

But seriously, the semiconductor has two layers, one of which provides excited 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!