11 Jun 2015

Color-Changing Beads: Fluorescence from UV Light

Category: ScienceAdmin @ 11:27 am

The usual explanation for the behavior of color-changing beads is that they change color when they absorb ultraviolet light. True enough, but why?

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The name for this process is “fluorescence.” Some pigments absorb light of one wavelength and quickly emit ‐ fluoresce ‐ light of another (or sometimes the same) wavelength. This is similar to the phosphorescence of glow-in-the-dark toys, clocks, etc., except that the light is emitted much more quickly in the case of fluorescence.

At the atomic level, when light hits an atom or molecule, an orbiting electron gets bumped up to a higher energetic level. In the simplest model, electrons are represented as particles orbiting the nucleus of the atom or, in the case of a molecule, the nuclei of the atoms that are bound together. When the light “particle,” called a photon, hits the atom or molecule, an electron gets bumped into a higher orbit. This is not the normal state, but rather an “excited,” higher-energy state, and it is unstable. Sooner or later, the electron gives off its excess energy and settles back down to its original, lower orbit. The excess energy can be given off as either light or heat, depending upon the type of molecule. When it is given off as light, we call this “luminescence.” There are, as previously mentioned, two types of luminescence: fluorescence, which happens right away, and phosphorescence, which happens more slowly.

Now we come to more refined questions arising from our original one: Why does light sometimes excite an electron, and why does an excited electron give off energy as light when it relaxes back into its lower-energy state (or alternatively, why does it give off heat)? I do not know.

It is great that it happens like this, because that allows us to write mathematical equations that balance in either direction:

[original electron state] + [light or heat] = [excited electron state]

Our question is one of many that science has not answered (such as, “Why does gravity work?”) Science does not have all the answers, but the expectation that it should misses the point. The point of science is to ask better and better questions. Great discoveries happen along the way, leading to ever more questions.

Let’s get back to the beads. UV color-changing beads have pigments that absorb ultraviolet light, thereby exciting their electrons, which then give off light in the visible spectrum as the electrons return to their original state. Different pigments give off different colors, some pink, some orange, some violet, some blue-green. When you put them in the sun, they not only turn pretty colors, but also provide a nice way to test the presence of UV light, lending themselves well to experiments that test UV-blocking potential of things like sun glasses and sunscreen. Of course, they make great jewelry too, especially for kids.

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