Does the phenomena of diffraction occurs also for water waves or only for light and matter? I learned in one of Walter Lewin's lectures about the quantum mechanical explanation of diffraction for light and matter. Namely, when the wave-particle passes through the oppening, its position spread is sharply peaked, so according to Heisenberg, its momentum must be widely spread, which explains why it appears that the small opening acts as a point-source. It also justify considering the "large" opening as an infinity of point-sources. So that's very nice for light and matter, but what about water? Why does the Huygen's principle work for water? Or is it the same reason?