1. The problem statement, all variables and given/known data Say I have a boundary between two dielectrics then it's easy to show using a gaussian pillarbox that: D(1)-D(2)=free surface charge density=s where D(1) is the component of the first medium normal to the surface. But suppose that there's nothing else apart from two infinite dielectrics with a constant free charge density between then, how would I work out what the actual values of D(1) and D(2) are rather than just the difference? 3. The attempt at a solution It seems reasonable that the D field should be the same on both sides so that D=s/2 but I'm not sure how I'd prove this? Any help appreciated.