A camera flash works by applying short bursts of high voltage to a type of gas in a glass tube. The electrical energy required is small, but it has to be given very fast because since the flash doesn't last long. An ordinary battery cannot give such a high voltage or give it so fast, so a photo flash capacitor is used. An electronic circuit increases the battery voltage, and charges a capacitor. When the button is pressed, the capacitor releases the stored energy in about a thousandth of a second and makes a bright flash.