The wave fronts from the ocean crash on these rocks, and there’s a narrow channel through them. Most of the energy is dissipated by the rocks, but a little gets through. That little bit, upon exiting the channel, “spreads out…” applying Huygen’s principle, the red arrows show how the exit of the channel, treated as the source of the waves on the other side, give the curved waves we observe. (I probably could have gotten a better diffraction pic but it was getting cold).

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