This paper is describing the fundamental notions of phase contrast method introduced by F. Zernike (Zernike 1934) and how it can be applied to coronagraphy. These techniques were first designed for optical surface testing and therefore their use as coronagraphic instrument is not straightforward. This paper is mainly describing the Foucault knife edge test, its later improvement by Zernike and Burch and their effects as coronagraphic concepts. A formal demonstration is given in the case of the Phase Knife Coronagraph.