Young's Slits

Introduction

Thomas Young showed the wave nature of light in 1801 with his famous double slit experiment. The experiment described here does not just replicate Young's experiment, although that is interesting in itself. This experiment goes a step further by performing it one photon at a time to show the interference of single photons. This is not original, it has been done many times before in laboratories and universities. It is quite a demanding project to take on at home though. This project consumed most of my spare time for a little over seven months.

Achieving single photon interference actually only took about three months. The reason why this project ended up taking so long is that I decided to make it more flexible, so that the relationship between wavelength, object and diffraction pattern could be investigated. I also decided that due its significance, I would make it available as a live interactive experiment. This experiment is now connected to the internet, and with Java installed, you can control the experiment and plot your own results. Due to Heisenberg's uncertainty principle, your set of results will be unique.

Why this Experiment is Significant

The results of this experiment go some way towards demonstrating that light can behave like a wave or particles depending upon the circumstances. This is known as wave / particle duality. I have come to realize that there is no argument whether light is made of waves or particles, it is both, and everything in between. Light is made up of parts called photons that behave unlike anything we experience in the everyday world. This is why we have no single analogy that matches how they behave. The wave and particle behaviours are just the two extremes. A photon can behave in a manner anywhere between these extremes depending upon the circumstances. In this experiment, each photon is shown to behave like a wave and a particle in the same apparatus.

Perhaps more disconcerting is that not only photons behave in this strange way. Everything does. Electrons, protons and even whole atoms can and do behave as waves. For example, you may have a picture in your mind of an atom consisting of a nucleus with electrons orbiting around it like planets. In reality the electrons fit into the atom as waves in resonance.

The Young's slits experiment has been performed using electrons and the same wave phenomenon are observed. The experiment is much more difficult to carry out with electrons, so I have used photons.

This experiment also allows the relationship between an object and its diffraction pattern to be explored. This gives an insight into how information about an object is carried by light that has interacted with it.

To fully appreciate this experiment, you require a little knowledge about diffraction and quantum physics. The following sections provide a basic introduction to these subjects with the minimum of mathematics.