# Diffraction Through a Single Slit

Diffraction also occurs when a wave passes through a *gap* (or *slit*) in a barrier. This is shown in the animation below (**Note:** Flash Player 7 required). Try dragging the slider to change the size of the gap. How does this affect wave diffraction? When does maximum diffraction occur? (Think about your previous findings on the diffraction of sound around an obstacle).

This animation is just a guide - below is a proper mathematical ripple tank simulation of waves passing through a slit. What difference does the length of slit make in terms of diffraction?

When the gap size is larger than the wavelength, the wave passes through the gap and does not spread out much on the other side. When the gap size is equal to the wavelength, maximum diffraction occurs and the waves spread out greatly - the *wavefronts* are almost semicircular.

One way to explain the effects of diffraction is to use a mathematical method invented by Christiaan Huygens (14^{th} April, 1629 - 8^{th} July, 1695); a Dutch mathematician and physicist.

Huygens argued that a wavefront could be modeled as a series of wavelets. A wavelet can be described as a circular- shaped wave much like the ripple you would get from dropping a small pebble into a pond. These wavelets superimpose and interfere to form more complicated wavefronts. For example - if you dropped a number of pebbles in a straight line, all 'in one go' at exactly the same time, a 'straight' (in science-speak *plane*) wavefront would be created.
The animations below show Huygen's principle in action:

This slow-motion video shows what happens when a single wavelet is created by a droplet falling into still water.

# Diffraction Through Two Slits

## Young's Experiment

So far we've only considered the case of a single slit or gap for the wave to pass through. What happens if there are two or more slits? We'll end up with two or more diffracting waves, which we might expect to interfere with one another...

Below in a simulation of diffraction through two slits. The experiment is named after the guy who first carried it out - Young's double slit experiment. Have a look at what is happening to the right of the slits. Is there a pattern? Is the amplitude larger at some places than others? You've seen this before on the Sounds Amazing site...

To the right of the slits, the waves interfere with each other. In fact, you can generate the same patterns by placing two sources where the slits are. The sound through each slit diffracts and radiates rather like two '*point sources*'. So the patterns you are observing are very similar to those for two *sources* whose wave radiation interferes together. You might want to have another look at the pages on interference - all the formulations and concepts are applicable to Young's double slit experiment.

Think back - if we are dealing with the interference of two sources, there will be places where the waves are in phase and cause constructive interference, and other places where the waves are out of phase and interfere destructively. In an audio example, the two slits could be replaced with two loudspeakers, and the maxima and minima in the wave superposition would then correspond to locations of loudness and quiet.

We'd hear these loud / quiet areas one after another as we moved in an arc in front of the loudspeakers - they're called '*Young's fringes*'. If the experiment is carried out using light waves, you get bright locations for constructive interference and dark locations for destructive interference. Young used this experiment to measure the wavelength of light.

In acoustics, the most common example of two sound sources interfering occurs when listening to stereo loudspeakers. If you have Java enabled and stereo speakers connected to your computer click here for an interactive experiment.