A Fabry-Perot cavity or Fabry-Perot interferometers is one of the fundamental building blocks of many laser interferometers. A Fabry-Perot cavity consists of two mirrors facing each other. The mirrors form an optical resonator in which a light field, e.g. a laser beam, can be resonantly enhanced. The notebooks in this directory explain and investigate some of the most important properties of a Fabry-Perot cavity, using a plane-wave analysis.