It was back in the 17th Century that Newton suggested that light should be treated as a stream of particles. Like many of his bright ideas, this one stayed around for a while. Until the Nineteenth Century, that is, when a former child prodigy showed that the particle nature of light could not possibly be right. Using two slits that were illuminated with light, Thomas Young showed that interference occurred. This is something that can be seen when water waves collide and create peaks and troughs or simply cancel out. Young had shown that light was a wave.

The world of physics did not have long to congratulate itself on this great discovery. James Clerk Maxwell, a Scottish physicist, had shown that visible light was a sort of electromagnetic radiation; just part of the spectrum that contained radio waves, x-rays, infra-red radiation and ultra-violet radiation. But, experiments carried out at the end of the Nineteenth Century were inexplicable using the wave theory of light. One of these experiments explored the photo-electric effect.

It seems that shining ultra-violet radiation onto a metal caused it to lose its charge, yet visible light had no effect. It was Albert Einstein who first came up with a theory to cover this so called anomaly. He suggested that the discharge was caused by electrons being knocked from the metal by little bundles of energy held within the radiation. These bundles, known as photons, had varying energies depending on the type of radiation. Ultra-violet photons therefore have more energy than visible light. It was out of this strange collection of ideas that "quantum" mechanics was born!

This article was first published in 1999