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Reflection of water waves at a coast is usually not an important part of their behaviour, unless the coast has a steep cliff or a seawall. However, reflected waves tend to interfere with the oncoming waves, and these patterns can be studied. When two waves travelling in different directions meet, they combine their energies and form interference patterns. This can result in regions of very high waves when they add up (constructive interference) alternating with regions of diminished or no waves when they cancel out (destructive interference). Refraction is the change in direction of a wave as it slows down. In shallower water near the coast, waves slow down because of the force exerted on them by the seabed. If a wave is approaching the coast at an angle, the nearshore part of the wave slows more than the offshore part of the wave (because it’s in shallower water). This is why the wavefront changes direction. Refraction is the reason why surf waves often line up parallel to the beach. Even if waves are coming in from deep water at an angle to the beach, the move to shallower water means that the waves will slow down and curve around (refract) so they are more parallel as the surf hits the beach. Refraction is very important for tsunamis because (unlike other waves) they interact with the seabed even in deep water – so they are always undergoing refraction. This affects the direction that the tsunami travels through the ocean. Tsunamis also refract around land masses.

Diffraction: when waves bend Waves that pass through small gaps (such as between the island and the mainland) diffract – they bend outwards. This can also lead to constructive and destructive interference patterns on the far side of the island. When waves get to a barrier such as an offshore rock or a small gap such as the opening to a harbour, they don’t go straight past the barrier or carry on straight after going through the gap. Instead, they bend – they curve outwards after passing through a gap and spread around an object. This is diffraction. It happens when the wavelength of the wave and the size of the gap or barrier are similar.