The Science of Optics: What is a Photon?

Once scientists started developing a better understanding of how light works - how it propagates and its effects. One of the biggest mysteries has been - what is it exactly? Is it a wave? Is it a particle? It turns out that it is in some ways neither, and in some ways both, and now I will explain why we know this.

The Wavelike properties of light

The wavelike properties of light are just like water waves - although they are not attached to a surface like water. They are transverse waves, which means the oscillation occurs at right angles to the direction the wave travels in, like in water, where the wave oscillates up and down, but the wave appears to travel along the surface. This is unlike sound waves which are longitudinal waves. In these sorts of waves, the oscillation occurs in the direction of the wave movement, like along a slinky spring.

How do we know that light is a wave? Well to test this, we can use a property known as interference. If we have two sources of waves that are correlated with one another - that is they are both of the same frequency and start off in phase, then there are a number of experiments that we can do. The simplest of these experiments is the Young's Double Slit experiment.

In this we have two slits that act as the sources and we shine light through them. The light is then projected from there on to a screen and we observe the pattern on the screen. When the path length is the same, the light arrives in phase and we get constructive interference - the two waves add up and we get a bright line. Where the path lengths are half a wavelength out of phase (in the water, this would be where a wave peak meets a wave trough) we get destructive interference and we get a dark line. If we know a few facts like the distance from the screen to the slits, the distances between the slits and the distance between the bright fringes, then we can do a bit of trigonometry and work out the wavelength. A more detailed explanation can be found on wikipedia's page on the Double-Slit experiment.

Interference can only occur where we have waves, so we know that light is a wave.

The Particulate nature of light

If light is a wave, then as the wave expands out into space, the energy will become ever thinner along with it, because energy is conserved. The wavelength however will change. For a water wave, the energy of the wave is related to its amplitude - the bigger the amplitude, the bigger the wave and the more energy. Just look out at sea - you need a pretty big wave with a lot of energy to throw an oil tanker around, but yachts can be thrown around by much smaller waves.

If we shine light onto a metal, then under certain conditions, electrons are thrown off the metal. This is because the electrons absorb the light, and if they have enough energy to escape the surface, they can jump off, in a sense like boiling a kettle where if the water has enough energy, it can leave the liquid and become a gas. This is called the photoelectric effect. Investigations into the conditions of the photoelectric effect threw up a rather strange phenomenon, and eventually it was Einstein who explained it.

It was noticed that if we shine really intense light on to a metal, and then measure the energy of the electrons coming off it, the energy of these electrons was the same as if we shone much dimmer light of the same frequency. The intensity is like the amplitude I mentioned earlier, so obviously this is pretty weird. Imagine our yacht going over a wave, for a really big wave, our unfortunate captain might be flung overboard, but for a little wave he can stay on the deck sipping his tea and enjoying the cruise, but for electrons, this isn't what happened at all. Regardless of the amplitude, the electrons were all flung from the surface with the same energy. What did change though, was the number of electrons that were thrown from the surface.

When the frequency of the light was changed, it was found that the higher the frequency, the more energy the electrons had when they were flung off. If the frequency was below a certain value, then no electrons would be thrown off at all, regardless of how bright the light was.

Einstein was the first to explain this in 1905, in his Nobel Prize winning paper "On a Heuristic Viewpoint Concerning the Production and Transformation of Light". The idea he presented, was that light came in discrete packets that he called "light quanta". The amplitude of light related to the number of these quanta, but the energy of each quanta was related to its frequency. Electrons could absorb one of these quanta and if there was enough energy to escape the attraction of the surface then they could, with the remaining energy going into the kinetic energy of the electron.

This explains the results perfectly - increased amplitude, or numbers of these quanta results in more electrons of the same energy being thrown off the surface, and increased frequency results in the electrons having more energy.

So we now know that light consists of quanta, or particles.

But hold on, we already said that light was a wave, and we demonstrated it with experiments. Now we are saying that light is a particle, and also we demonstrated it with experiments. Scientifically we can alter the theory, but we can't just toss out facts we don't like, so what is going on here? Enter Quantum Mechanics.

In Quantum mechanics, things may have both wavelike and particulate properties. It doesn't really make sense to ask whether something is a wave or a particle. Those after all are just descriptions of things that we are familiar with, that have certain sets of properties. Quantum Mechanics shows us that all things have properties that are both wavelike and particle like. I'll stick with photons here, since that's what I'm talking about, and it's what this blog is all about, but the best way to describe this, is that photons propagate in a manner that is wave like (they spread out over space), but interact in a way that is particle like (they interact discretely). This of course raises loads more interesting questions, like "well where is the photon if it is spread out all over the place but only interacts once, and in one place" but for now, this will have to do. If you do want to confuse people though, if someone happens to ask "what is a photon" a good concise description of it would be "a quantized excitation of the electromagnetic field" - I will get on to the electromagnetic field another time.

A much more in depth explanation of the photon and the history of the photon can be found in W.E Lamb's paper "Anti Photon, Applied Physics B, vol 60 p77-84 (1995).