Carl Potton, of Port Chalmers School, asks :-

How do mirror's work?

John Campbell, who teaches solid state physics at the University of Canterbury, responded.

You have posed a tricky question. First we need to know something about atoms. Everything is made up of atoms, each of which is so small that it would take about five million side by side to cross a full stop on a printed page. Each atom consists of a small central core, which contains most of the mass, surrounded by much lighter electrons. It is the interaction between the incident light and these electrons which determine proprties such as the reflection of light.

In a material which doesn't conduct electricity, such as glass, the electrons cannot leave the atom. The glass is then transparent. We can see the surface of the glass because whenever a light wave moves from air into glass it slows down. In doing so some light is reflected. The more the wave speed slows down then the more the amount of light reflected. For light shining directly onto glass, only about four parts in every hundred of the incident light is reflected.

More efficient mirrors are made using polished metals. The ancient Greeks polished the surface of a metal called speculum. Nowadays, the easiest and cheapest way to produce a mirror is to coat a flat glass surface with a very thin layer of silver or aluminium metal. A metal conducts electricity because one or two electrons per atom are free to move away from the atom and move throughout the metal. These electrons oscillate in step with the oscillating ight wave and re-radiate the light. (Much like electrons oscillating up and down a radio aerial radiate radio waves). A clean metal mirror reflects about ninety-eight of every hundred parts of incident light so are very efficient.