We have already considered various different situations where waves are reflected (sound waves incident on a rigid wall, elastic waves incident on a free surface and electromagnetic waves incident on perfect conductors and insulators). Whilst pure reflection is characterised by the presence of a uniform, infinitely long barrier, at which the wave changes its direction of propagation, scattering is characterised by the presence of a smooth, finite body, which leads to a non-uniformly reflected wave field. In this chapter, we consider barriers to wave propagation that are semi-infinite or finite. We will find that, as well as reflection and scattering, there is another effect caused by the presence of an edge to a barrier. When the incident wave impinges upon the edge, it is scattered in all directions. This is called diffraction and can produce some interesting effects. If there are two or more edges, the scattered waves from each edge can interfere with each other to form a diffraction pattern. Although reflection, scattering and diffraction are treated using slightly different mathematical methods, the underlying idea is that of the interaction of an incident wave with a boundary. The reflected/scattered/diffracted field is the difference between the unaffected incident wave and the actual solution.

Situations where these effects are important are widespread in the natural world, in the laboratory, in engineering applications and in the home. The sky is blue because of the way sunlight is scattered by air molecules.