Look at the glow from a neon striplight and what you are seeing is the light output of billions of individual neon atoms. Neon produces this light because it is being held in the form of a plasma.

You might remember learning that there are three states of matter: solid, liquid and gas. It’s much less common to encounter the fourth state, which is plasma; yet 99.99% of the matter in the observable universe exists in this form.

The Sun [Image: NASA/JPL] Copyrighted image Credit: NASA/JPL For example stars, like our Sun, are composed of plasma.

What is a plasma?

Atoms are normally electrically neutral, consisting of identical numbers of positively charged protons and negatively charged electrons. In a plasma some of the electrons have been separated from their atoms and are free to wander. The atoms which have lost an electron (these are referred to as ions) are still present and preserve the overall neutrality of the plasma.

Atoms, electrons, ions Copyrighted image Credit: OU
Atoms showing atoms in green, with electrons in blue and ions in red.

In a star, the separation of the electrons is achieved through heating. However, it is possible to make a plasma that is cold. The presence of free charges means that, unlike a gas, a plasma will conduct electricity. In fact the plasma is formed and preserved by placing a voltage across the gas.

The presence of a voltage causes the free electrons to be accelerated, and they collide with neutral atoms turning some of them into ions, thus sustaining the plasma.

What causes the plasma to glow?

Many of the collisions between atoms and electrons don't result in ionization. Instead, some of the energy from the collision is absorbed by the atoms before being swiftly re-emitted as invisible ultra violet rays and visible light with colours that are specific to the particular species of atom.

This is what you see when you look at a neon lamp. The distinctive orange-red colour of the glow from neon is a characteristic of the element neon, other gases, such as argon, would glow with a different colour.

This is not the whole story; the tubes that James examined in the TV programme are coated with a material that 'fluoresces', converting invisible ultra violet into visible light. By doing this it is possible to vary the colour of the lamp, but that's another story...