Compact fluorescents (CFLs)-energy-saving, weird-looking, or ugly. No matter what you think of them, their physics gives them efficiency.

In fluorescent lights, electrical current doesn’t run through a metal, but goes through gas confined in a tube. Ordinarily gas can’t carry an electrical current because all the electrons are tied to their atoms. So the first thing a fluorescent lamp does is inject some electrons into the tube, then speed them up with high voltage. That flow of electrons surges through the tube, hitting against other atoms.

Just as in a solid, some of the energy of the electronic current goes into heat, but the voltage puts the electrons into a narrow energy range, and because the gas is thin—not dense like a metal filament—the electrons stay in that narrow range as they fly along. The narrow range can be tuned to transfer just the right amount of energy to a specific atom, so very little is wasted.

In fluorescent bulbs the specific atom is mercury, and the “right amount” of energy puts a mercury atom into an excited state. That mercury electron returns to its lower energy state by emitting a photon. (That’s why it’s not so easy to dim a fluorescent bulb: turning down the voltage changes the speed of the electrons rushing through the gas, which messes up the way they’re “tuned” to the mercury atoms, which pretty much shuts the bulb down.) While in incandescent bulbs most of the photons are in the invisible infrared wavelengths, mercury atoms emit their photons in the invisible ultraviolet region of the spectrum. To turn those invisible photons into visible light the inside of a fluorescent tube is coated with phosphor.
Originally published at Suite101, 29 MAR 2011