The Double Slit Experiment

Big Questions

Thomas Young was the first to perform the double slit experiment around 1805. His aim was to determine whether light behaves like a particle or a wave.

The Basic Setup

A beam of light is shone through two very narrow slits placed a certain distance apart in an otherwise solid barrier. On the other side of the barrier there is a screen where we can see the resulting light pattern.

What We Expect

The patterns on the other side will reveal if light behaves like a particle or a wave. If light behaves like a particle, then it must travel in a straight line. On the other side of the barrier, we would expect to see the individual beams of light as they travel through the slits. Plain and simple.

If, on the other hand, light behaves like a wave, we expect to see something called an interference pattern. Interference patterns happen when waves meet eachother and combine their energies.

All waves have peaks which are the high points, and troughs which are the low points. So, when two waves meet, some of the wave energies cancel eachother out because the peaks in one wave meet the troughs of the other. When peaks meet peaks and troughs meet troughs, the wave energy is added together to create an extra big peak or an extra big trough. This interference pattern in the case of light will represent itself as alternating bright and dark spots.

Results!

When the slits are are a good distance apart, we see what we would expect to see in either case. The light has made it through these two slits and we see two vertical sections of light on the other side. They are not close enough to interact.

Now, move these slits closer together. When they get close enough, the pattern on the other side changes a bit. The bands of light have now combined in a way that shows alternating bright spots and dark spots. The crests of the light waves have added together to create bright spots and the troughs have added together to create more bright spots. The dark spots result from the crests of one wave and the troughs of the other wave canceling each other out. This alternating light and dark pattern is the interference pattern. So light behaves like a wave!

Another Wave Demonstration

A nice way to see interference patterns in action is by making a similar set up with water.

Send a wave of water towards a divider with two spaces for the waves to get through. On the otherside, the ripples come through at the two different points and these two new ripples interfere with eachother! This is the same thing that is happened with our light experiment.

A third wave demonstration

Another example of wave interference is in a vibrating string. The string vibrates not just one way, but in harmonics (you can learn about these on the gamma page). The string vibrates along its entire length, but also in halves, in thirds, in fourths... etc. The different harmonics vibrate in different waves, and the sum of these different waves dictates how the string vibrates.

In the animation on this site: 'Standing Waves', you can see how this principle works. The white string is the sum of all the colored strings, and illustrates how the waves cancel each other out or make each other stronger.