Here and below we keep to a definition of the reflectivity coefficient of the mirrors that implies that the reflected wave
acquires a phase shift equal to
with respect to the incident wave if the latter impinged the reflective surface from the less
optically dense medium (air or vacuum). In the opposite case, when the incident wave encounters reflective surface from inside
the mirror, i.e., goes from the optically more dense medium (glass), it is assumed to acquire no phase shift upon
reflection.