Quicktime movie illustrating this aspect of diffraction.) But if the wavelength is large compared to the post, then the waves begin "lapping around" the post and start to fill in the space behind it. In other words, a sharp shadow is created behind the post. ![]() If the wavelength is very small compared to the post, then the oncoming waves act more-or-less like particles, in the sense that points on the wave which just miss the post keep on moving, whereas points on the wave which impact the post are stopped. When a wave moving in two or three dimensions runs into an obstacle (let us say, a water wave striking a post sunk into a harbor), what happens depends on the wavelength of the wave. ![]()
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