Because it is a basic property of every process involving vibrations, interference has been introduced as early as the first chapter of this book. Furthermore, the notion of interference is usually generalized to other domains, quite often far from Physics, such as Economy or Psychology . Two phenomena are said to interfere if their simultaneous actions have new consequences, and if compared with the superposition of the consequences of their independent actions.

General conditions for two light beams to interfere have already been given: they should have exactly, and with high accuracy, the same frequency; their polarizations should not be strictly orthogonal and last but not least, they must be coherent with one another. The fact that two light beams, coming from two independent sources, cannot interfere is rather a good thing, if this were not the case everyday life would occur in a vast field of interference where everything would be striped with interference fringes.

All the experimental set-ups that we are going to describe are supposed to work with incoherent sources. To give the photodetectors the impression that they receive coherent beams, we will always use arrangements in which all the different interfering beams originate from a unique point source.