1. The problem statement, all variables and given/known data I was a asked a question from an experiment: What happens if the light shone into the diffraction grating or a double slit is not perpendicular to the diffraction grating or double slit? What will happen? 3. The attempt at a solution Straight away i can tell that the light might not arrive coherently to the slits because of this. But i don't know how incoherent light coming out of the slits will effect anything. All i can imagine happening is that the interference pattern will just shift, so lines of reinforcement will not appear where they were before, but the fringe separation (for double slit) will not be effected at all? Am i right? Or wrong?