These are pictures of Flodigarry Island off the north end of Skye taken from the Quaraing. The diffraction of the waves bothered me and I have had to do a lot of research to explain it. I teach that waves will only diffract around an object if their wavelength is the same size as the object. That's why FM radio reception is poor in mountainous areas. But look at the waves in the picture. Their wavelength is tiny in comparison to the length of the island and yet they diffract round both ends of the island. The answer is called the knife-edge effect and owes a lot to Huygens' understanding of how waves propagate. He came up with idea that you divide the wavefront up into points that act as sources of semi-circular waves. Where these semi-circles overlap, you get the new wave. On a plane wave (a flat fronted wave) the points are so close together that the result is actually another plane wave. But a knife edge can act as a distinct secondary source of waves with none inside it. The key to my example is that the island is not round. It has pointed ends and these pointed ends are about the same width as the wavelength. So the ends of the island act as secondary sources, diffracting circular waves into the space behind the island. A mountain does not act as a knife edge to FM radio waves. The size of the mountain is too big for that.
