Zone-plate Lens for Focusing of Ocean Swells
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Zone-plate lenses can also be used to focus water waves. A lens for water waves works in a similar manner as an optical lens: as the wave passes over a submerged body (e.g. a lens element), it is retarded because it moves slower in shallow water than in deep water. By adjusting the shape and depth of the various lens elements, one obtains the intentional transformation from a diverging to a converging wave. The idea of focusing ocean swells for power production purposes was proposed by Mehlum, and has been the subject of many investigations in recent years. Heier and Stamnes (1982) computed the focused field using a combination of ray tracing and diffraction theory. First they employed ray tracing to compute the amplitude and phase of the field right behind the lens, and then they used diffraction theory to compute the field everywhere on the image side of the lens.