When light originating from a medium of lower refractive index reflects off a surface of higher refractive index, why does the phase shift by 180 degrees? Stokes relations show that reflection coefficients at the surface are equal but of opposite signs depending on direction of light (i.e. from lower to higher refractive index versus higher to lower); but Stokes relation does not tell me explicitly which one is shifted by 180 degrees. Can someone please provide an explanation for this. I have researched the internet with no success. Thank you in advance.