Megan O'Brian, age 12, of Springfield, Mass., for her question:

WHAT MARES A FLASHBULB WORK?

A flashbulb is a convenient source of artificial light used to take pictures indoors or take outdoor pictures at night. A flashbulb also often improves the quality of pictures taken in bright sunlight by removing dark shadows.

The bulb is either placed in a holder attached to the camera or held nearby on an extension cord. A reflector behind the bulb helps to direct the light.

The holder contains batteries or a small generator, which provides an electric current to ignite the bulb. The camera shutter has built in electrical contacts to fire the bulb when the shutter is wide open during the exposure.

The flashbulb contains a fine tungsten wire filament, a zirconium primer and finely shredded metallic foil. Electric current heats the filament which ignites the primer. The burning primer then ignites the foil.

Because the lamp is filled with oxygen, the burning foil produces an intense white flame that varies in time and intensity with the type of bulb used. Flashbulbs are rated and numbered according to their size and burning characteristics.

The bulb is filled with aluminum or zirconium foil. Zirconium produces a brighter flash than aluminum, so bulbs using zirconium are smaller than aluminum foil bulbs of equal light output.

The burning within the flashbulb takes place so rapidly that it creates heat and a small explosive force. A lacquer coating on the inside and outside of the lamp helps absorb the heat and also prevents the bulb from shattering.

A cracked bulb that allows some air to enter will not fire properly and may explode. Some lamps have a spot that changes color to indicate that air has leaked in.

Most flashbulbs give the greatest light in one fiftieth of a second.

Speed midget, or SM bulbs, contain no foil but have a larger primer. This type produces an extremely bright light for a short time. It burns only the primer in an oxygen filled bulb.

The flash of light in an SM bulb lasts only one two hundredth of a second.

A blue coated lamp used with ordinary daylight type color film makes it possible to take color pictures indoors.

Another type of bulb is coated with a special lacquer that holds back all visible light rays and permits only infrared rays to light the subject. When used with a film sensitive to infrared rays, pictures can be taken in complete darkness.

Electronic flashlamps are gas filled tubes that require a high voltage power source. They can be fired several thousand times.