Does a light wave in air (n1 = 1) that is reflected off a glass surface (n2=1.5) experience a 180deg phase change? Looking at the Fresnel equations (theta = 0deg) I learn that: according to http://en.wikipedia.org/wiki/Fresnel_equations rp = (n2-n1)/(n1+n2) = 0.2 and rs = (n1-n2)/(n1+n2) = -0.2 rp indicates no phase shift, while rs does indicate at 180 deg phase shift (rs is negative). Intuitively, the phase shift should not depend on polarization at normal incidence (theta = 0deg). How do I understand the above formulas? What if the polarization is circular?