Standing waves vibrate at a certain frequency but do not move along the medium. The vibrations of guitar strings and violin strings are standing waves. Standing waves can also occur on a surface, such as a drumhead, or in an enclosed space, such as a pipe of a pipe organ. Changes in wave motion Wave motion changes when waves move from one medium to another and when one wave meets another. Refraction and reflection.
Refraction is a change in wave direction when waves pass from one medium into another.
When waves leave one medium and enter another, some of the energy in the waves is reflected and some is transmitted to the new medium. It also depends on the density and other properties of the two mediums. The direction taken by the transmitted wave, called the refracted wave, also depends on the angle of incidence and the characteristics of the two mediums. Diffraction is the spreading out of waves as they pass through an opening or by the edge of an obstacle. An expanding ring of water waves moves away from a stone dropped into a pond.