Refraction

Practice

Waves travel in all directions in the open ocean, but they always approach the land nearly perpendicular to the shore. Why does this happen?

Ocean waves normally form when the wind grips the surface of the water and tries to drag it along. The friction at the interface gives the water a little tug and piles it up into a wave. Short burst of wind make little ripples and strong, steady winds make larger waves or swells. Regardless of size, waves generated by this means generally propagate in the direction that the wind is blowing. Since the wind can and does blow in every direction, waves can and do travel in any direction when they are formed.

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The speed of an ocean wave is affected by the depth of the water through which it is propagating. As the sea floor approaches the shore it rises and depth decreases. The shallower the water, the slower the wave speed. (The relationship is a complex one, but near shore speed is approximately proportional to the square root of depth.) Waves entering a medium with slower wave speed are refracted towards the normal. Where the sea floor rises suddenly, the refraction is abrupt. Where it rises gradually, the refraction is gradual. The closer a wave gets, the more perpendicular its propagation. The result is that most waves near the shore will eventually wind up heading very nearly perpendicular to the shore no matter what direction they were traveling in initially.

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