You may think these tubes are empty, but you’re wrong. The tubes contain special gases. They do not react easily with other chemical substances. We also call them noble gases. Normally, they are invisible. But look what happens when you turn a tube in the device… It glows!
The device is a Tesla coil. It generates a very high voltage which causes an electrical discharge: a current flows through the tube. This is how the noble gas particles receive a lot of energy. They soon lose that energy again; the coloured light shows this.
Did you also notice that each gas glows in a different colour? Neon glows orange-red, argon makes pink,… this is how you can recognise a gas. Sometimes scientists need to investigate a gas in a lab to find out which one it is. They will discover this by the colour(s) of the light it emits.
Noble gases in illuminated advertising
I’m sure you know what a neon lamp is? It is a glass tube that is filled with a noble gas. You would think that a neon lamp is always filled with neon, but strangely enough, this is not always the case. Sometimes it is also filled with other (noble) gases like argon or mercury.
Just like in the setup, such a lamp emits light because an electric discharge takes place in it. Depending on the gas (or gases), the lamp has a different colour.