Phase of R and T as a function of the incidence angle, θ_I, for different values of the density contrast, ρ_c. The vertical dash-dotted, dotted, and dashed lines correspond to the Critical angle, θ_C, the Brewster angle, θ_B, and the Phase inversion angle, θ_P, respectively, introduced in Sect. 3. The black dashed horizontal lines correspond to the phases 0 and π. The blue dashed horizontal line denotes the phase difference equal to π/2 and one can see that its intersection point with ϕ_R defines the Phase inversion angle, θ_P.