Science In Real Life

Today Science In Real Life takes a turn for the tacky with a foray into neon lights. Strange as it may seem, something as commonplace as a lit tube of neon gas relies entirely on quantum mechanical phenomena to function. If I may soapbox for a moment here, this is why funding pure research is important: quantum mechanics was once the most obscure, arcane theory ever devised by mankind, and within a few decades it gave rise to the very practical field of electronics, and the slightly less practical field of neon light fixtures.

So, before we get to the good stuff, a quick review of how non-neon lights work. Incandescent light bulbs, the old standard, are very simple: a thin filament of (usually) tungsten in a glass bulb with no air inside it. Remember from the essay about electricity that in a wire electrons are more or less flowing through. In a copper wire they can do this easily, we say that a copper wire has low resistance. Tungsten, on the other hand, has high resistance, so pushing current through it is about as easy as pushing fur coats at a PETA convention.

So when you do finally sell the coat to someone, there’s a big commotion – releases a lot of energy. Everyone, meaning all the tungsten atoms, gets very heated. When tungsten atoms get heated enough, they glow. If you’ve ever peeked into your toaster while it’s browning your bread, you’ll see the same effect, only with a lot less visible light and a lot more heat. Every material has what physicists call an emission spectrum – how much light of what frequencies it will give off at a given temperature. It so happens that tungsten gives off visible light when heated to the temperatures provided by household electric current.

So what’s going on in a neon light? There’s no filament. Like Jumpin’ Jack Flash, it’s a gas. A little bit of chemistry first – and here is where we start to brush up against some quantum mechanics, so be ready. Neon is what’s called a noble gas, not because it sacrifices of itself to give to those less fortunate, but because like nobility of ages past it refuses to associate, i.e. have chemical reactions with, all the other elements (excepting extreme conditions). The whole righthand column of the periodic table is referred to as the noble gases. Why does it do this? The answer to that question is related to how we make it light up.

Atoms, remember, are made up of a positive nucleus and a cloud of electrons. Take a look at this, the common illustration of the structure of an atom:

(Image credit)

Very nice, makes for some pretty corporate logos. But it’s horrendously inaccurate. Electrons don’t have well-defined orbits; they do not circle the nucleus as planets do a star. In fact, and here’s where it gets really weird, electrons don’t even have a well-defined position. (And you thought pandering politicians were bad.) What they do have is regions where they are and are not allowed to be – the allowable regions are called energy levels, because each one corresponds to how much energy an electron has while there. This is at the heart of quantum physics. The energy that an electron can have is not arbitrary or continuous, it is discrete and quantized.

Because each element has a different number of protons in its nucleus, the pattern of available levels is unique to each element. If you study how these unique patterns progress as you add more protons and neutrons, you can see trends. These trends are displayed in the layout of the periodic table – I’d even say there’s more information in the layout than is written in the individual squares. But right now our concern is simple. In order to be electrically neutral, atoms need to have the same number of electrons as protons. Each energy level can hold a certain number of electrons, and it so happens that when the noble gases are electrically neutral, their outermost level is “full”, like a psychiatrist’s schedule right after one of the SAW movies comes out.

So neon and the other noble gases don’t want to exchange electrons with any other atoms, because then they won’t have a nice complete set, and they’ll be electrically imbalanced. Let’s get to the heart of the matter – how do we make the pretty lights? Remember that the energy of an electron is associated with which region it’s in. We simply connect both ends of the tube holding neon gas to a source of electricity. When a current flows through the gas, it imparts energy to the electrons in the gas, bumping them up to regions of higher energy. When they drop down again to a lower region, that energy has to go somewhere. It is emitted as visible light. Just like a stone on top of a rise. When I roll the stone down the hill, the energy that was stored as keeping-it-high-up is released.

Why neon? Why not argon or xenon? Well, it so happens that the energy level structure of neon permits lots of different colors depending on how much electricity we use, much more than other noble gases. And here comes the surprise – fluorescent lights work using this same principle, but with an intermediate layer of phosphorous. Atoms of mercury vapor, excited by electricity, emit light in the ultraviolet range, which then hits the phosphorous and excites THAT, and then it emits in the visible range. Like an outfielder that always hits the cutoff. Home run!

And that’s the last time I try baseball analogies. G’night folks.