Hello, I'm a photonics student at UCF and I ended up here while trying to understand why diffraction works. I wanted to add my interpretation to the thread and see if it's correct and maybe it can help someone else also. I think I got it. It comes down to the distribution of potential energy. In a water wave that passes through a small inlet, the potential energy of the wave is the gravitational potential energy. Say the wave is completly calm on one side and a wave comes in from the other side. After the energy passes through the inlet the energy "wants" to spread out, because that's just what energy does. It's one of those laws of thermodynamics. #BasedEntropy Anyway, we usually think of gravitational energy spreading to the sides because the earth is "pulling" it down from the sides. You can convince yourself of that by imagining what would happen if diffraction of a water wave wouldn't happen. There would be a wave with a width the same as the width of the slit going up and down and on either side the water would remain still. At the high points it would be far higher than the calm water on either side. Once you get that image in your head you can see why it wouldn't happen. Gravity would pull the high water in all directions away from it's crest. In a light wave, the potential energies that we're talking about are the electical and magnetic potenial energies that make up the wave. So diffraction should happen there as well. But now I'm thinking there's really no potential energy in an electric field unless there is a charged particle. Clearly light has some potential energy, but I'm not sure about where it "lives" in the light.