Diffraction of waves

Imagine that some (blue) waves were washing against a (red) barrier. If a bit of the wave went through a hole in the barrier it would be like a spike of water in an otherwise calm place. That would not be stable, the spike would collapse. It collapses outwards in a semi-circle.

Diagram explaining that a spike of water could not be stable when a wave meets a narrow gap in a barrier.
This does not happen to waves

Labelled diagram showing that the waves diffract in a semi circular pattern through a narrow gap
Diffraction of waves through a narrow gap

For the spreading (diffraction) to be semicircular the gap has to be a similar size to the wavelength.

Arial shot of water waves diffracting through a gap
Water waves diffract through a gap


It is not just water waves that do this but all waves. That is why we can hear sound around the corner of a building or a doorway or over the top of a wall.

A lady shouts and the sound waves spread out to reach a man because Sound waves diffract around a corner
Sound waves diffract around a corner

Some of the energy of the sound waves diffracts around the top of the wall. However the shorter the waves the less they diffract. The lady can be heard from over the wall (by the way, the man thoroughly deserves to be shouted at) but she cannot be seen. That is because the waves of light are very short and the amount they diffract around the wall cannot be seen.

Light will diffract noticeably through very small gaps, this can be seen in Young's slits experiment or you can try yourself with a tiny pin hole in a sheet of aluminium foil.

Some light will reach the eye even when the hole, the eye and the lamp are not in line. That is diffraction.
A simple demonstration of the diffraction of light.

There are more pages on the properties of waves, here: