A
flash that uses a very fast and intense
electric discharge in a
rare gas (usually
xenon) to generate a very intense and short-lasting flash of light.
The electronic flash was invented by Harold "Doc" Edgerton to do high-speed photography, and its basic principle is as such:
In an electronic flash you have two capacitors, that are charged by a battery. When you shoot the picture, the first, small capacitor discharges through a glass tube filled with xenon: the discharge ionizes the gas, making it conductive. That's when the second, bigger and meaner, capacitor discharges through the ionized path. The xenon, having been electrically abused, glows for a time on the order of the thousandth of a second (and usually shorter). The intensity of the light source is extremely high (in fact, it compares with the sun), but the duration is extremely short (otherwise your flash would melt).
When to use it
When you want to freeze motion. When you need a light source that has a comparable
color temperature to the sun. When you need a portable, high-intensity, light source that can run from small batteries.
The popular misconception that flash light is "hard" is, in fact, a misconception: any point light source produces hard shadows. Photographers make it softer by using either reflectors or gobos.
Types
Well, there is the
studio flash, the
built-in flash and, for lack of a better name, the garden-variety
portable flash (the kind that you can attach and detach to your camera, usually on the
hotshoe).