When does a phase change of π occur in EM wave reflection?

[h0dgey84bc]

Hi,

When exactly does the phase change of $$\pi$$ occur when a EM wave reflects from a surface? I notice that it happens in Lloyd's mirror for example, and also when considering non reflective coatings on lenses. Is it just when the EM wave is going into a medium with higher refractive index or is there more to it than that?

Thanks

 

[Doc Al]

Is it just when the EM wave is going into a medium with higher refractive index or is there more to it than that?

For waves with normal incidence, that's all there is to it. But generally the phase change depends upon angle of incidence and polarization (whether the wave is polarized parallel or perpendicular to the surface).