How does this change for real waves? What I mean by real waves is something like water waves, where there is actually something that is vibrating, so that the sources on the wave-front are real: do these sources generate secondary waves that are isotropic?
Take 2-slit diffraction of water waves. The screen blocks the waves, but the slits can be seen as sources for a new wave because there is actually water at the slits that vibrate (like dipping your finger in a pond at the slits). Do the water at the slits generate backwards waves?