2. The first two-wave interferometers

Young's interferometer is particularly instructive for understanding the phenomenon of light interference and the problems of spatial and temporal coherence, but it has the major disadvantage of considerably limiting the extent of the source. The source has to be finer than the interferometer, which leads to very low luminosity setups.

To observe bangs with an extended source, we need to work with localized bangs, i.e. bangs that can only be observed where the difference in travel between the interfering rays depends little or not at all on the position of the source. The Michelson and Fabry-Perot interferometers show perfectly localized bangs: rings at infinity. But before these interferometers existed a number of set-ups, some of which are still in use today.