All wave phenomena matching the conditions   There was brief mention of analogous results for acoustics but it would be nice to have an expansion of that The most general result on the page has to do with Helmholtz theorem and when combined with Greens identity that applies to ALL wave phenomena not just EM which match the boundary conditions linear water waves gravity waves acoustics etc If I have time Ill point that out Unfortunately itll probably require radically expanding the whole article and certainly renaming it to Reciprocity wave phenomena  If someone else can do the necessary editing thatd be great too