16.13 : Standing Waves

Sometimes waves do not seem to move; rather, they just vibrate in place. Unmoving waves can be seen on the surface of a glass of milk kept in a refrigerator, which is one example of standing waves. Vibrations from the refrigerator motor create waves on the milk that oscillate up and down but do not seem to move across the surface. These waves are formed or created by the superposition of two or more identical moving waves in opposite directions. The waves move through each other, with their disturbances adding as they go by. If the two waves have the same amplitude and wavelength, and the oscillations are at fixed locations in space, then they alternate between constructive and destructive interference. The resultant waveform looks like a wave standing in place and, thus, is called a standing wave. Some other examples of standing waves include those resulting from plucking the strings of a musical instrument, such as guitar strings, or blowing an organ pipe and are formed due to reflections of waves from the ends of the string or the organ pipe. Nodes are the points where the string does not move; more generally, nodes are where the wave disturbance is zero in a standing wave. The fixed ends of strings must be nodes, too, because the string cannot move there. The word antinode is used to denote the location of maximum amplitude in standing waves.