Bright Bulbs

Q: Why do light bulbs get real bright just before they burn out?
Teddy, age 10, Albuquerque, New Mexico

A: A normal incandescent light bulb contains about a meter of fine tungsten wire, wound cleverly to fit into the glass bulb and to function properly, says Louis A. Bloomfield, professor of physics at the University of Virginia. When the bulb is on, an electric current flows through the filament from one end to the other. The electrons making up this current have energy, both in their motion and in the forces they exert on one another. The electrons collide with the tungsten atoms as they flow through the wire.

Consequently, the tungsten atoms and the filament get extremely hot, typically about 2500 degrees Celsius. About 10% of the energy radiated is light and, therefore, the tungsten filament glows white-hot.

As the bulb emits light, tungsten is constantly burning off the filament. The tungsten evaporates from the solid wire, like ice evaporates from frozen clothes, but not uniformly. Eventually the wire gets thinner in spots. Since the whole wire carries the same current, the thinner parts get hotter than the rest of the wire.

A thin spot has less conducting material, therefore a higher resistance. Because of its higher resistance, it dissipates more power and gets hotter. At this higher temperature the thin areas evaporate at a still higher rate until the bulb fails.

You cannot turn off current instantaneously, however. When the wire burns out, the electrons arc through the metal vapor.

The bulb is filled with argon gas. The electric spark flashing through the gas ionizes the argon and "the resulting gas discharge sometimes emits a brilliant blue-white light", says James D. Hooker, editor of Lighting Equipment News in the United Kingdom.

Further Reading:

How things work: the physics of everyday life"" by Louis A. Bloomfield

Philips, The Light Site, Global

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