power is supplied to a ballast, which is basically a transformer that changes the 12 volts supplied by the vehicle to about 10000 volts. the bulb itself consists of two electrodes separated by a small space surrounded by a gas such as xenon. when the charge is passed through the electrodes, the electrons in the atom of the gas go to a higher energy level and quickly return down to a lower energy state, thus giving off photon energy in the process.