THE EXPLAINER WHAT IS LIGHT?

IS LIGHT A WAVE OR A PARTICLE?

Neither: light is its own unique phenomenon – the outcome of an interaction between electrical and magnetic fields – and it behaves like both waves and particles.

Most of us were taught at school that light is a wave. This is because it does things that waves do. So, for example, waves undergo interference. To understand this, imagine that you’ve just dropped two stones in a still pond. Each of those stones will create ripples, with the water creating waves that move up and down. Eventually ripples originating from each stone will hit each other, and the waves travelling in the same direction at that point (up and down, rather than the same direction across the water) will become stronger – this means if a wave peaking upwards hits another wave peaking upwards, it will grow stronger. Equally, those waves travelling in opposite vertical directions could cancel out each other’s movement. This is interference – and the same thing happens with light.

However, the quantum revolution of the 20th Century made it clear that light also behaves as if it were a stream of particles. Einstein realised that this was necessary for the photoelectric effect – the mechanism behind solar cells – to work. And our entire understanding of the interaction of light and matter requires light to act like particles known as photons.

Perhaps the easiest way to think of light is as a stream of particles, but quantum particles, which are unlike ones that we can see. And all quantum particles, whether they are photons, or matter particles such as electrons, have wave-like behaviour.