Neon lamps

by Chris Woodford. Last updated: June 1, 2011.

What makes night-time cities fizz and crackle with life? Brightly colored neon lamps play a huge part. If you've ever seen the lights dancing in Tokyo, New York City, or London, you'll know exactly what I mean. Whole streets seem to leap alive the minute the neon switches on. Strictly speaking, lamps filled with neon gas can make only red light and you need other gases to make other colors. In fact, by mixing different gases, it's possible to make over 150 different colors of "neon" light—and paint the night sky with almost any color you like! Let's take a closer look at how these things work.

Photo: Neon lights in the Shinjuku district of Tokyo, Japan. Photo by Lynn Jenkins courtesy of Defense Imagery.

How atoms make light come to life

If you've read our article on light, you'll know that atoms produce light when they absorb energy and become "excited". In their excited state, they're also unstable—so they rapidly give out the energy they absorbed to get themselves back to normal again. They do this by giving out tiny packets of light energy called photons.

You can use this idea to make an electric light. Suppose you fill a tube full of atoms and seal it up at both ends. Now put some sort of electrical device inside the tube that can keep feeding energy to the atoms. When you switch on the power, the atoms will constantly get excited and give out light. That's pretty much how a fluorescent lamp works—and it's how a neon lamp works as well. (Let's also note in passing that it's how a laser works too, although in a laser the outgoing light is made into a super-concentrated beam.)

Neon display showing stars and stripes in Tokyo Closeup of neon display showing stars and stripes in Tokyo
Photo: Neon lights in the Shinjuku district of Tokyo, Japan. Look closely and you'll see that each "stripe" in the display is made from a separate glass tube, a bit like the fluorescent strip lights you might have in your kitchen or classroom. The "stars" contain single tubes that have been heated, bent round into angles while hot, then allowed to cool again. You can make all kinds of letters, characters, and other shapes by bending tubes in this way. Photo by Lynn Jenkins courtesy of Defense Imagery.

How neon lamps work

Artwork showing how a neon lamp works

A neon lamp is a sealed glass tube filled with neon gas, which is one of the so-called "noble" (inert or unreactive) gases on the far right of the Periodic Table. (There are minute quantities of neon in the air around us: take a deep breath and you'll breathe in a volume of neon as big as an orange pip!)
There are electrical terminals at either end of a neon tube. At one end, there's a negative terminal ("-ve", shown blue); at the other end there's a positive terminal ("+ve", shown green).
When the tube is switched off, it contains ordinary atoms of neon gas (brown circles).
Rig the terminals up to a high-voltage power supply (about 15,000 volts—because you need a lot of "electrical force" to make things happen) and switch on, and you'll literally start pulling the neon atoms apart. Some of the atoms will lose electrons to become positively charged ions (big green dots). Being positively charged, these neon ions will tend to move toward the negative electrical terminal.
The electrons the neon atoms lose (small blue dots) are negatively charged, so they hurtle the opposite way toward the positive terminal at the other end of the tube.
In all this rushing about, atoms, ions, and electrons are constantly colliding with one another. Those collisions generate a sudden smash of energy that excites the atoms and ions and makes them give off photons of red light.
So many collisions happen with such rapidity that you get a constant buzzing of red light from the tube. You also get quite a lot of energy given off as heat. If you've ever stood near a neon light, you'll know they can get very hot. That's because the atoms are giving off quite a bit of invisible infrared radiation (in other words, heat) as well as visible radiation (better known as red light).

Why is neon light red?

Artwork showing quantum leaps in neon and argon

The energy levels inside atoms are a bit like rungs on a ladder or steps on a staircase. Electrons can only be on the rungs or steps, not on the gaps in between. That means atoms can absorb or release energy only in fixed-sized packets (called quanta, which is the plural of quantum) and atoms of different chemical elements will give out quanta that are bigger or smaller, depending on their precise inner structure. Atoms that give out bigger quanta of energy make higher-frequency (bluer) light than atoms that give out smaller quanta. In neon, the quanta of energy that are given out correspond exactly with light that we see as red. Other noble gases make light of different colors. Argon, for example, makes blue light—so when you see "neon" lamps shining blue, you're actually looking at tubes filled with argon, not neon. You can make different colors by putting more than one gas in the same "neon" tube. To make green tubes, you need neon and argon together. For purple, you'd use argon and xenon.

Photo: When electrons in neon atoms return from their "excited" state to their "ground" (unexcited) state, they give out packets of energy called quanta that our eyes see as red light. In argon atoms, the quanta are bigger and our eyes see them as higher-frequency blue light.