Monday, February 23, 2009

Why do white clothes glow under blacklight?

I haven't posted in a month! I'm sorry for the hiatus. I wish I could say that I've been getting ready for my big move, but I still haven't done much. I have a week longer than I thought I would though, so I have a little time yet. I'll be leaving on March 6th now.

I recently went to a show with a few friends. While the bands were playing, most of the normal lighting was replaced by blacklight, so our white clothes were glowing, including Katie's skirt:

She asked me why white clothes glow under blacklight and said that I should write about it here, so that's what I'll do.

Let's review some basic physics/chemistry first. Electrons in an atom have some base amount of energy. An electron can gain energy from some outside source (e.g. light or electricity), putting it momentarily into a higher energy state. The electron can then lose this energy by emitting light - more specifically, by emitting a photon (light particle) that has this exact amount of energy. This energy corresponds to some wavelength of light. This is, for example, how a neon sign works. The electrons in the neon gas are excited into a higher energy state by electricity, then they lose the energy and "fall back down" to their lower energy by emitting light (predominantly orange light in neon's case).

As I mentioned above, an electron can be excited by light as well as electricity. When an atom absorbs a photon, the energy of the photon gets transferred to the electron. The electron can then lose the energy by re-emitting a photon of the same energy. In other words, the incoming photon and re-emitted photon have the same energy.

And now I can confuse you and tell you that this is not always the case. In an atom, these electron transitions are the main way for energy to be gained and lost, but in a molecule there are other ways. A molecule consists of multiple atoms and the bonds between them. When a molecule gains energy, the increase in energy can still manifest itself in electron transitions OR in rotations of the molecule OR in vibrations of the bonds between the atoms. This means that when a molecule absorbs a photon, not all the energy has to go into one type of motion - for example, vibrations and an electron transition can both occur on the absorption of a photon. The vibrations of the molecule can die out quickly (mainly because it runs into the other molecules around it), but the excited electron still has to fall back down. But since some of the energy has already been vibrated away, the re-emitted photon will have less energy than the incoming photon, which corresponds to a longer wavelength of light. (For example, a molecule might absorb blue light but emit green light.) This process is called fluorescence when the absorbed light is in the visible or ultraviolet (UV) range.

Back to the question at hand: Why do white clothes glow in blacklight? Blacklight is mainly just UV radiation. If you've been to a natural history museum, you may have seen certain minerals glowing under blacklight in an otherwise dark case. The glowing clothing is the same thing... but why just white? White cloth and paper are actually treated with a special substance (an optical brightener) to make them look even whiter. The brightener absorbs UV radiation and re-emits blue light, which balances/hides any yellow or brown tones in the cloth or paper. This is not obvious under normal lighting because the cloth is mostly reflecting visible light; the optical brightener is just a small enhancement. But when there is little visible light and an abundance of UV radiation, the brightener has a lot to absorb (and not much to reflect), and the re-emitted blue light is very obvious. This blue light is the glow!

You may have also noticed that teeth glow under blacklight too, but they glow a slightly different color. This is not because of any sort of brightener. The chemical makeup of teeth simply causes them to fluoresce greenish under UV radiation.

Katie, I hope this helps! Keep the questions coming!

I usually like to find better sources than wikipedia, but it has some good information in its "fluorescence" article.