Why Do Neon Signs Glow Different Colours?

Neon signs glow because high voltage excites gas atoms, and each gas gives out its own colours. See what neon, argon, helium and more look like.

A neon sign glows because electricity pushes through a sealed glass tube of gas and excites its atoms, which then give out light as they settle down again. Each gas gives out its own set of colours, so neon glows red-orange, while other gases, coloured glass and glowing coatings supply the rest of the rainbow. In short: it is a flame test that has been given a mortgage and a nice bit of glassblowing.

How does a neon sign actually work?

Start with a glass tube, bent into letters, a squiggle or a very ambitious cocktail glass. Pump almost all the air out. Let in a small amount of one gas, so the pressure inside is far lower than the air around you. Seal both ends, each with a metal electrode (a metal contact that lets electricity in and out).

Now switch on a high voltage. Voltage is the push behind an electric current, and a sign needs a big one, typically thousands of volts, which is why a sign has a hefty transformer tucked away somewhere. The tube needs that much because the gas is a poor conductor to begin with. The push has to be strong enough to knock a few electrons loose from atoms, and once that happens, the tube starts to conduct and everything gets lively.

Here is the plain version of what follows:

1. The loose electrons are shoved along the tube by the voltage.
2. They crash into gas atoms on the way. Each crash can hand an atom's own electrons a small packet of energy.
3. An electron that gains energy jumps to a higher energy level. Scientists call that being excited.
4. It cannot stay up there. It drops back, and the energy it gives up leaves as a tiny packet of light.

That is the whole trick. No burning, no filament, nothing getting hot enough to glow like a toaster.

Why does each gas glow its own colour?

Electrons in an atom can only have certain amounts of energy. These allowed amounts are the energy levels, and you can think of them as the steps of a staircase: you can stand on step three or step four, but never on step three and a half. Our guide to atoms has the longer story, and electron configuration shows how electrons are arranged.

The size of the drop between steps decides the colour of the light. Light travels as a wave, and its colour is its wavelength, the distance from one crest to the next: longest for red, shorter for blue. A big drop gives bluer light and a small drop gives redder light. Every element has a staircase with steps at its own heights, so each one can give out only its own set of colours, a bit like a fingerprint made of light. Often an atom gives out several colours at once, and your eye blends them into one overall glow.

If this sounds familiar, it should. It is exactly the idea behind our guide to flame tests and firework colours, where a hot flame supplies the energy instead of an electric current. Same staircase, different way of shoving electrons up it. Sodium burns golden yellow in a flame, and the same sodium glows that familiar orange-yellow in a street lamp, for the same reason.

What colour does each noble gas glow?

These descriptions come with a hedge, because colour is judged by eye. The brightness, the pressure, the width of the tube, the glass and even a trace of impurity all nudge the result. So treat the colours below as the ones people most often describe.

• Neon: a warm red-orange. It is the colour most people picture when they hear "neon sign", and the glow is bright and strong.
• Helium: often described as pinkish or a warm white, sometimes with a peachy tint. It is usually a dimmer, softer glow than neon.
• Argon: a pale lavender or violet-blue, and rather gentle on its own.
• Krypton: often described as a pale whitish glow, sometimes with a greenish tint.
• Xenon: a pale blue to lavender.

These five are all noble gases, the far-right column of the periodic table, and we will get to why they are the popular choice in a moment. If you want to compare neighbours, there are pages for helium and neon and for argon and krypton.

Is a "neon sign" really neon?

Often, no. "Neon sign" has become the catch-all name for any glowing tube sign, a bit like calling every vacuum cleaner a Hoover. Proper red-orange signs are neon, but plenty of the other colours come from other tricks.

Argon with a little mercury. Many bright blue signs contain argon plus a small amount of mercury vapour. Argon helps the electricity get going, and the mercury atoms then give out a strong ultraviolet glow and some bluish light of their own. Mercury is the only metal that is a liquid at ordinary room temperature, and a tiny droplet in the tube turns into vapour as it warms. A blue sign of that kind is, strictly speaking, not an argon glow at all but a mercury one, with argon acting as the stage crew.

Phosphor coatings. A phosphor is a powder that absorbs invisible ultraviolet light and re-emits it as visible light. Paint the inside of the tube with one and the ultraviolet becomes a colour of your choice: whites, greens, golds, pinks and many more. This is the same idea as an ordinary fluorescent strip light in a school corridor, where mercury vapour makes ultraviolet and a white coating turns it into the grim glow of double maths on a Monday.

Coloured glass. Tinted glass filters the light, letting through only some of it. Put clear red-orange neon behind blue glass and you will see something very different from the bare gas.

So the honest truth is that a sign called neon may hold neon, argon, mercury, a phosphor coating, tinted glass, or a mix. "Neon" is a convenient shorthand. A nice one, though, and the name stuck because neon was the star of the early shows.

Who first made neon lights, and when?

The gas itself was found first. Neon was discovered in London in 1898, and our data credits Morris Travers (working with William Ramsay), who isolated it from air. The name comes from a Greek word for "new", which was a fine thing to call something that had been hiding in the air all along.

The showman was the French engineer Georges Claude. He worked with industrial gases, and neon turned up as a by-product when air was cooled and separated. Claude is usually credited with putting it into a glass tube, applying electricity and showing neon lighting to the public in Paris in 1910. Soon after, neon tubes were lighting up shop fronts, and the idea spread worldwide.

Why are noble gases used in signs?

Because they do not react. The noble gases are famously unreactive: each atom has a full outer shell of electrons, so it has little interest in sharing, swapping or stealing electrons with other atoms. (A shell is a group of energy levels, and our guide to valence electrons explains why a full outer one makes an atom so aloof.)

That matters in a tube for two reasons:

• The electrodes last. A reactive gas would attack the metal contacts, especially when they are being bombarded by fast-moving particles.
• The glass stays clean. The gas does not form coatings, crusts or new compounds on the inside of the tube.

They are also not flammable, and most can be got from air. A very low-drama gang.

Why are most signs LEDs now?

An LED (a light-emitting diode) makes light in a solid chip rather than a gas. Electricity pushes electrons across a junction between two kinds of material, and light comes out as they settle. The colour depends on the materials in the chip, and white or other colours can be made by adding coatings.

Plenty of modern signs that look like neon are LED strips inside a flexible, glowing tube. The reasons are practical:

• They use much less electricity for the same brightness, and they run at low voltage.
• They are tougher and easier to fit. Glass tubing bent into letters is fragile and needs a skilled craftsperson, while LED strips shrug off knocks.
• They offer any colour. One LED set-up can change colour on demand, which a gas tube cannot.

Real glass neon has a warm, buzzy, slightly imperfect glow that LEDs imitate rather well but never quite match.

The noble gases at a glance

Here are the noble gases with their atomic number (the number of protons in each atom), the glow most often described in a sign tube, and one other everyday use.

The last member of the family, oganesson (atomic number 118), has only ever been made a few atoms at a time, so nobody is putting it in a window. Radon is radioactive (see which elements are radioactive), so it stays out of lighting. Compare xenon and radon if you like.

All of these are gases at room temperature, as this guide to gases, liquids and solids explains.

Try it on Periodic Bubbles. Open the bubbles and choose Colour: category from the colour menu (on a phone, tap Display first), then find the far-right column: those are your noble gases. Pop neon and argon and see how the pages describe them, then try xenon for a heavier cousin.

Why do the tubes need high voltage?

Your torch runs on batteries, so why not a sign? A torch has a solid wire to carry the current, whereas a sign tube starts with a gas that has almost no free electrons in it at all. Electricity needs charged particles to travel through, and a cold, thin gas simply has very few of them lying about.

A high voltage creates a strong electric field, which gives any stray electrons a hefty shove. Those electrons knock further electrons off atoms, which knock off more, and the tube quickly becomes a conductor. This chain reaction is called ionisation, which means turning atoms into charged particles (ions) by removing electrons. Once the tube is conducting, the sign draws a modest current, but it takes that big initial push to get going.

A safety note. Sign transformers produce dangerous voltages. Signs are installed and repaired by trained professionals, and a broken tube should be left for them to deal with, particularly as some contain a little mercury.

Frequently asked questions

Which gas is in a neon sign?

Red-orange signs contain neon. Other colours might contain argon (often with a little mercury vapour), helium, krypton or xenon, or a mixture, and phosphor coatings or coloured glass can change the colour further.

Why does helium not make a bright sign?

Helium does glow, usually a pinkish or warm white, but it tends to be a softer, dimmer glow than neon.

Are neon signs dangerous?

The gas inside is not the problem. The danger is the high voltage that powers the sign, plus the small amount of mercury in some tubes. That is why repairs are for professionals, not for a weekend and a screwdriver.

Do all noble gases glow?

Under the right conditions, the ones you can actually handle can all be made to glow in a tube. But radon is radioactive, and oganesson has only ever been made a few atoms at a time, so in practice only helium, neon, argon, krypton and xenon turn up in lighting.

Fancy testing yourself on glowing gases and other chemistry? Take the Pop Quiz, or see what element the Daily Bubble has picked for you today.