Hello! I´m trying to read Jacksons 'Classical Electrodynamics' and solving some problems. At the moment I´m stuck at problem 7.3. I started looking at suggested solutions (http://www-personal.umich.edu/~pran/jackson/P505/F07_hw11a.pdf) but I need some help I guess. Looking at how other people have done the boundary conditions at the first interface point (medium to air) with an electric field polarized perpendicular to the plane, they set up the condition of continuity as: E_1 + E_2 = E_3 + E_4. Where E_1 is the incident wave, E_2 the reflected, E_3 the transmitted, and E_4 the reflected wave of the second interface. However the wave E_4 is never actually present at the first interface point! Why should we include this wave in the equation? It is reflected at the second interface point and then travels toward the medium again but at a distant point. Intuitional it seems to me that the boundary condition should be: E_1 + E_2 = E_3. Where does my logic fail? Thanks in advance!