Glow, little glow-worm,
shimmer, shimmer . . .

Have you ever wondered why glow-worms shimmer?

Probably not, especially if, like me, you wouldn’t know a glow-worm from a tapeworm and wouldn’t care to meet either one.

But maybe you’ve seen fireflies dancing in the dark, or, on a more practical note, been thankful for the little glowing spots on your old-fashioned analog alarm clock that tell you it’s three o’clock in the morning when you’re awakened by a strange noise.

All of these are examples of luminescence, a phenomenon that is of great interest to scientists and great practical use to everyone else.

Luminescence is defined as “light emission that cannot be attributed merely to the temperature of the emitting body.” Almost anything will glow if it gets hot enough, unless it oxidizes (burns) first; the point is that luminescent substances glow even when they aren’t particularly hot.

Luminescence is named according to what triggers it. If it comes from a chemical reaction, it’s called chemiluminescence. If it occurs in a living thing (such as a glow-worm, presumably), it’s called bioluminescence.

Then there’s cathodoluminescence, caused by the bombardment of electrons (as in your television set), radioluminescence, caused by the bombardment of X or gamma rays, photoluminescence, caused by the bombardment of visible, ultraviolet or infrared light, electroluminescence, caused by the presence of an electrical field, etc., etc., etc.

These classifications are convenient, but they don’t really establish a distinction among the various kinds of luminescence. The bioluminescence of a firefly, for example, is caused by chemical reaction and is therefore also chemiluminescence.

Another way in which luminescence is sometimes divided is into fluorescence and phosphorescence. Basically, an object is fluorescent if it glows while being excited by some form of energy, but stops as soon as the energy is withdrawn. It is phosphorescent if it continues to glow for a while after the energy input ceases.

Even this isn’t a real distinction, because it depends on how sensitive your measuring instrument is. If an object glows for a fraction of a second after the energy input ceases, is it “merely” fluorescent, or is it weakly phosphorescent? Hmm?

The more precise definition of the two terms is that an object is fluorescent if its afterglow is independent of temperature and phosphorescent if its afterglow decreases with increasing temperature. To understand why that is a real distinction, you have to understand why things glow in the first place.

(By the way, the “glow” doesn’t have to be visible light. X-rays are a form of luminescence created by bombarding a metal screen with an electron beam, and most of us are familiar with “black light” lamps that luminesce in ultraviolet.)

Any state of matter can luminesce, but the most basic example, and the easiest one to explain, is an isolated atom of a gas. This lonely atom can exist only at specific levels of energy. The lowest level, the most stable and the one it’s usually at, is called the ground state.

In fluorescence, this atom is briefly excited by the absorption of energy–say, the electrical field in a neon light–but because that higher level of energy is unstable, it quickly falls back to the ground state, releasing energy in the form of a photon. If the energy that excited it to begin with is still around, it gets excited again and falls back again, releasing another photon each time, over and over and over. But once the excitation ceases (the light is turned off), the atom falls to its ground state and can’t get up–and no more photons are emitted.

However, in phosphorescent substances (called phosphors), there is an intermediate state between the ground state and the excited state, called a metastable state. An atom may stay in this state for an indefinite period of time, and cannot fall from there to its ground state. In order to get back to the ground state it first has to absorb enough energy to go back up to its excited state, from which it can fall all the way back to ground state, emitting its photon as it does so. It is this waiting around in the metastable state that makes the afterglow last so long in a phosphor.

One way an atom can be boosted from the “in-between” metastable state is by colliding with another atom. Hot atoms move around more than cold atoms, so in a hot phosphor, you get a bright but short afterglow as all the metastable atoms are quickly boosted back up to the excited state and then fall to ground state, while in a cold phosphor you get a long afterglow of low intensity.

Now the heat-related distinction between fluorescence and phosphorescence makes sense. (I hope.)

Very few pure solids are luminescent; most of those used commercially are created by adding substances called “activators,” (manganese, for example), which confer luminescent properties on the material into which they are placed. This is usually done by grinding the host and the activator into very fine powders and mixing them under high temperatures.

There are also substances called “poisons,” mainly from the iron-nickel-cobalt group, which can quench luminescence.

Luminescent substances are all around us, in everything from TV sets to neon signs to X-ray machines to lasers. The study of luminescence also gives insight into the structure of various materials. It really is a broad and fascinating field.

In fact, I get a warm glow just thinking about it.

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