How Everything Works
How Everything Works How Everything Works

Question 1052

How does a strobe light work? — JM, Kettering, OH
A strobe light passes a brief, intense pulse of electric current through a gas, which then emits a brilliant burst of light. The gas is usually one of two inert gases, xenon or krypton, that emit relatively white light when they're struck by the fast moving electrons in the electric current. When it hits a xenon or krypton atom, an electron may give up some of its kinetic energy—its energy of motion—to the electrons in the atom. Those atomic electrons shift from their usual orbitals (quantum mechanically allowed orbits) to higher-energy orbitals that they usually don't travel in. The atomic electrons remain only briefly in these higher-energy orbitals before dropping back to their original orbitals. As they drop back down, these electrons give up their extra energy as light. Because krypton and xenon atoms have a great many electrons and their electronic structures are very complicated, they emit light over a broad range of wavelengths. Moreover, the gases are at relatively high pressures and collisions between the atoms while they are emitting light further smooth out the spectrum of light they produce. Thus the strobe emits a rich, white light during the moments while current is passing through the gas.

Supplying the enormous current needed to maintain the brief arc in the strobe's gas is done with the help of a capacitor, a device that stores separated electric charge. A high voltage power supply pumps positive charge from the capacitor's negative plate to its positive plate, until there is a huge charge imbalance between those two plates. You can often hear a whistling sound as this power supply does its work. The capacitor plates are connected to one another through the gas-filled flashlamp that will eventually produce the light. However, current can't pass through the gas in the flashlamp until some electric charges are injected into the gas. These initial charges are usually produced by a high voltage pulse applied to a wire that wraps around the middle of the flashlamp. When a few charges are inserted into the gas, they accelerate rapidly toward the positive or negative wires that extend from the charged capacitor. As these charges pick up speed, they begin to collide with the gas atoms and they deposit energy in those atoms. Electrons are occasionally knocked out of atoms or out of the wires at the end of the flashlamp and these new charges that enter the gas also begin to accelerate toward the wires. A cascade of collisions quickly leads to a violent arc of charged particles flowing through the flashlamp and colliding with the gas atoms. The flashlamp emits its brilliant burst of light that terminates only when the capacitor's separated electric charges and stored energy are exhausted.


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