Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)

How do you make a fluorescent light?

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

Now, if the inside of the tube is coated with glow-in-the-dark phosphors that absorb the ultraviolet light, they will emit exactly the colors we want. Little bits of each color of phosphor can added to tune the light to be any shade of white (or any other color) desired. The tube can simulate sunlight or get a bluer or redder light to suit the mood.

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.