Several interferometric set-ups using virtual sources, namely Fresnel’s mirrors, Fresnel’s biprism, and Lloyd’s mirror, are described and analysed using the same approach as Young’s experiment. The Rayleigh interferometer is also presented as an example of interferometry based on the wavefront-division technique. Several interferometers based on the amplitude-division technique, namely the Michelson interferometer, the Twyman–Green interferometer, the Mach–Zehnder interferometer and the Sagnac interferometer, are analysed, and some of their applications are discussed. Interferometers using optical fibres are also described. Finally, the Fabry–Perot interferometer is analysed using the results obtained in Chapter 8 for the case of multiple-reflection interference. The main parameters characterizing the Fabry–Perot interferometer are described.