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HOW COME?: What makes ordinary lightbulbs shine

Kathy Wollard

September 29, 2008

How come ordinary lightbulbs shine? asks Benjamin Heo.

Lightbulbs take advantage of the fact that excited, energized atoms emit particles of light when they settle back down. But before there were electric bulbs, there was fire. For thousands of years, people lit up the darkness with campfires and torches, candles and oil lamps, and, finally, gas lights.

Electric lightbulbs let people see what was in front of them on a dark night, without flames, fumes, smoke or soot. In 1879, American inventor Thomas Edison built a lamp that used thin strands of carbon sealed in a glass bulb. The bulb, emptied of air by a vacuum pump, was hooked up by wires to an electric current.

When Edison turned up the juice, the carbon filaments heated up - and began to glow. Supplied with a steady current, the filaments glowed white-hot for 40 hours, until they finally disintegrated. The lightbulb Edison is credited with inventing is called "incandescent," the kind still found in most household lamps and lighting fixtures. The word "incandescent" means "to glow with heat."

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Today's incandescent bulbs work in much the same way. In the metal base, two metal contacts make contact with wires in the lamp socket, which are, in turn, connected to the household current. Metal wires lead from the contacts to the center of the glass bulb, where they hook up to a thin metal filament.

The filament is made of tungsten. Tungsten can be heated to a higher temperature than Edison's carbon without disintegrating. The glass bulb surrounding the filament is sealed off from the room air, since oxygen promotes burning. Instead of oxygen-rich air, the bulb is filled with a gas like argon and/or nitrogen, which helps preserve the about-to-be-heated filament.

Like the coiled cord connecting a telephone receiver to its base, the tungsten filament is much longer than it appears. Uncoiled, the thin metal thread would extend more than 20 inches. Coiled, and then coiled again, it fits neatly into the center of a small lightbulb.

When the current is switched on and electrons begin moving, tungsten atoms in the filament get energized. The atoms' energetic vibration creates heat. In fact, according to University of Virginia physicist Louis Bloomfield, the temperature of the filament in an average 60-watt bulb soars to more than 4,500 degrees Fahrenheit!

As the temperature rises, atoms get more energetic. And when individual atoms fall back to their ordinary energy levels, they release the pent-up energy in the form of photons of light. The result: We can see to read on a moonless night.

But the very thing that creates the light we see by - heating metal atoms to high temperatures - makes incandescent bulbs inefficient at producing light. According to Bloomfield, a 60-watt bulb produces only about 6 watts of visible light. The other 54 watts are wasted as heat energy. This explains why a 40-watt bulb, emitting only a dim glow, is still too hot to touch. It also explains why even low-wattage bulbs help eggs hatch in a chicken incubator.