NA 4. Diffraction

All types of waves can be diffracted. Diffraction is where waves spread out as they pass through a gap (or aperture) or when they pass close by an object.

There are two general rules for diffraction:

• • Diffraction through a gap - the waves are diffracted through a bigger angle if the gap size is close to (or smaller than) the wavelength of the waves. The level of diffraction can be altered by changing the wavelength of the waves or by changing the gap size.

• • Around obstacles (essentially half a gap)  long wavelengths are diffracted more than short wavelengths. This means that long wave radio waves are diffracted more by obstacles like hills than the longer wavelength TV waves. This allows the radio waves to be picked up better at the bottom of valleys.

Sound waves often have a wavelength of around about a metre so they can be diffracted substantially by gaps around this kind of size. A doorway is around a metre wide so sounds will get diffracted around that gap and spread out on the other side. This is one reason why you can hear around corners.

Light waves have such a small wavelength that it is unusual to observe them being diffracted. It can happen if you have a very small gap though, and the diffraction pattern produced by sending laser light through a narrow slit is shown below: