So far only linearly polarised light has been considered. It occurs very commonly in both science and nature but it is actually a special case of a more general system which also includes circular and elliptical polarisation. Examples of these are much rarer but are of considerable interest anyway. In most cases the circular polarisation is derived from linearly polarised light which is itself very common. The theory may seem a little abstruse at first, but it is worth following in order to understand the larger picture.