 In order to specify the phase shift on reflection, we must choose a sign convention for the "reflection coefficient", which is the ratio of the reflected amplitude to the incident amplitude.In the case of the "s" components, for which the incident and reflected vibrations are both normal (perpendicular) to the plane of incidence, the obvious choice is to say that a "positive" reflection coefficient, corresponding to "zero" phase shift, is one for which the incident and reflected fields have the same direction (no reversal; no "inversion").In the case of the "p" components, this article adopts the convention that a "positive" reflection coefficient is one for which the incident and reflected fields are inclined towards the same medium.