You have seen toys that glow in the dark, and you have seen fluorescent colors in clothing and highlighting markers that seem to glow even when it isn’t dark. These objects glow because they contain compounds called phosphors.

A phosphor absorbs light of one color, and that energy is stored in the molecule (we say that the molecule is excited), and a little bit later the energy is released again as light and heat. Since some of the energy is lost as heat, the light that comes out has less energy than the light that went in. Light with less energy is redder in color.

So, to make a fluorescent light, we want to start with light that has a lot of energy (light that has a shorter wavelength). If light has a lot of energy, it will move so far in the blue direction that it goes past violet into the ultraviolet, which we cannot see.

A fluorescent light is a tube of glass that has a little bit of mercury in it, and very little else (almost a vacuum). When we put electricity into the mercury vapor, it gets excited and emits ultraviolet light, as well as some green and blue light, and a little bit of red.

Now we can coat the inside of the tube with glow in the dark phosphors that absorb the ultraviolet light, and emit exactly the colors we want. We can add little bits of each color of phosphor, to tune the light to be any shade of white (or any other color) that we want. We can simulate sunlight, or get a bluer or redder light to suit our moods.

Since the mercury vapor emits mostly light, and very little heat, compared to heating up a tungsten filament until it glows white hot, fluorescent lights use less energy than incandescent lights of the same brightness.