Diffraction
If a small obstacle is placed in the path of the ripples, and a slow frequency is used, there is no shadow area as the ripples refract around it, as shown below on the left. A faster frequency may result in a shadow, as shown below on the right. If a large obstacle is placed in the tank, a shadow area will probably be observed.
If an obstacle with a small gap is placed in the tank the ripples emerge in an almost semicircular pattern. If the gap is large however, the diffraction is much more limited. Small, in this context, means that the size of the obstacle is comparable to the wavelength of the ripples.
See also: Huygen's principle
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