Refraction

Refraction is the change of wave direction as a wave moves into water of different depth, where the shallower part of the crest slows and the crest turns toward the depth contours. It follows Snell’s law, sin(theta)/c constant along a ray, the same relation that bends light between media of different speed. For ocean surface waves the controlling variable is depth through the celerity c = sqrt((g/k) tanh kh): as h falls, c falls, and the crest rotates. Refraction redistributes wave energy alongshore, focusing it on headlands and dispersing it in embayments, and combines with shoaling to set the height a wave reaches at a given depth.