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 two animations below. The difference between the movies is the size of the gap.

When the size of the gap changes, how does this affect wave diffraction? When does maximum diffraction occur? (Think about your previous findings on the diffraction of sound around an obstacle).

Gap width = half a wavelength i.e. slit is narrower than the wavelength
Gap width = two wavelengths i.e. slit is wider than the wavelength

When the gap width is larger than the wavelength (bottom movie), the wave passes through the gap and does not spread out much on the other side. When the gap size is smaller than the wavelength (top movie), more diffraction occurs and the waves spread out greatly – the wavefronts are almost semicircular.

Huygen’s Principle

One way to explain the effects of diffraction is to use a mathematical method invented by 17th century physicist Christiaan Huygens.

Huygens argued that a wavefront could be modelled as a series of wavelets. A wavelet can be described as a circular 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 video below shows how you can use this method to work out how wavefronts are altered by a slit.

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 is 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? What creates this? Is the amplitude larger at some places than others? 

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. This video below nicely demonstrates this using water waves on a pond.

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.

Next: Diffraction grating

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