Waves Changing Direction

why do straight line waves change direction while passing over a shallow ridge that is parallel to the wave?
- Mike Fidell
Ellwood City, Pa.,USA.

Hi Mike,

It sounds as if you are working with water waves in a shallow tray. If your straight line waves impinge directly on a ridge that is parallel to the wave, then they really shouldn’t change direction at all, because of the symmetry of the situation. If they turned to the right, then you could make the same argument that they should have turned left.

If the ridge is not infinite in length, however, the edges of the waves will curve around the ends of the ridge and fill in the space behind the ridge. This is a phenomenon called diffraction.

If the ridge is not parallel to the wave crests, then the wave will change in direction because the speed of the wave in the shallow water will be less than it is in deep water.
See  for a description of the speed of water waves in different depths. This phenomenon is pretty much the same thing that happens to light when it changes direction upon entry into a material in which light travels at a different speed (refraction).

Tom

(published on 10/22/2007)