Electronic flash units are fairly simple devices, consisting of a capacitor and a gas-filled tube. Batteries provide the power, which is stored in the capacitor until the shutter is snapped. Then, an electrical charge travels through the tube and makes the gas inside it glow bright white.

At the same moment that the flash fires, the camera's shutter opens up, so that light from the subject (a mix of existing light and the flash) can flow through the lens and onto the film. As soon as the capacitor recharges — usually in just a couple of seconds — the flash is ready to fire again.

The burst of light made by an electronic flash unit lasts only about 1/1,000th to 1/10,000th of a second. This makes flash great for delivering sharp photographs because it eliminates blur caused by motion — your own or the subject's. The light created by an electronic flash also has the same color balance as daylight, which is an important consideration when you're using color film.