A "normal light bulb" is also known as an "incandescent light bulb" (see this page for a nice illustration). These bulbs have a very thin tungsten filament that is housed inside a glass sphere. They typically come in sizes like "60 watt," "75 watt," "100 watt" and so on.

The basic idea behind these bulbs is simple. Electricity runs through the filament. Because the filament is so thin it offers a good bit of resistance to the electricity, and this resistance turns electrical energy into heat. The heat is enough to make the filament white hot, and the "white" part is light. The filament literally incandesces because of the heat.

The problem with incandescent light bulbs is that the heat wastes a lot of electricity. Heat is not light, and the purpose of the lightbulb is light, so all of the energy spent creating heat is a waste. Incandescent bulbs are therefore very inefficient. They produce perhaps 15 lumens per watt of input power.

A fluorescent bulb uses a completely different method to produce light. In a fluorescent tube there are electrodes at both ends of the tube and a gas containing argon and mercury vapor within the tube. A stream of electrons flows through the gas from one electrode to the other (in a manner similar to the stream of electrons in a Cathode Ray Tube -- see How Television Works for details). These electrons bump into the mercury atoms and excite them. As the mercury atoms move from the excited state back to the unexcited state, they give off ultraviolet photons. These photons hit the phosphor coating the inside of the fluorescent tube, and this phosphor creates visible light. It sounds complicated so lets go through it again in slow motion:

  • There is a stream of electrons flowing between the electrodes at both ends of the fluorescent bulb.
  • The electrons interact with mercury vapor atoms floating inside the bulb.
  • The mercury atoms become excited, and then when they return to an unexcited state they release photons of light in the ultraviolet region of the spectrum.
  • These ultraviolet photons collide with the phosphor coating the inside of the bulb and the phosphor creates visible light.
The phosphor fluoresces to produce light.

A fluorescent bulb produces less heat, so it is much more efficient. A fluorescent bulb can produce between 50 and 100 lumens per watt. This makes fluorescent bulbs 4 to 6 times more efficient than incandescent bulbs. That's why you can buy a 15 watt fluorescent bulb and it can produce the same amount of light as a 60 watt incandescent bulb.

Here are several interesting links: