Ondes stationnaires

A standing wave forms when two waves with identical frequency and amplitude travel in opposite directions and interfere with each other. When a traveling wave meets a boundary, it reflects back, creating this interference pattern. The resulting wave pattern remains stationary along the string while its amplitude fluctuates, producing the characteristic up-and-down vibration seen in interference and superposition of waves.

Nodes are points where the sine function equals zero, resulting in zero displacement and no movement of the string. Antinodes are points where the sine function reaches its maximum value, corresponding to maximum displacement. In standing waves on fixed strings, the ends must be nodes because the string cannot move there, while antinodes represent the locations of greatest oscillation between these fixed points.

The standing wave wavefunction combines a sine function, which represents the sinusoidal simple harmonic oscillation, with a cosine function that acts as a scaling factor modifying the wave's amplitude. The sum of the individual wavefunctions of the two opposing traveling waves produces this combined wavefunction. This mathematical representation allows prediction of displacement at any point and time along the standing wave.

The two waves are in phase when time equals an integral multiple of half a period, resulting in maximum constructive interference and maximum displacement at antinodes. They go out of phase for an integral multiple of one-fourth of a period, producing destructive interference. This alternating phase relationship creates the characteristic stationary pattern of standing waves.

Standing waves appear in vibrating guitar strings, organ pipes, and even on milk surfaces in a refrigerator. These waves form due to reflections from the fixed ends of strings or pipes. The vibrations oscillate up and down at fixed locations in space without traveling across the surface, making them visible examples of superposition and interference in everyday settings.

Standing waves are created by the superposition of two or more identical moving waves traveling in opposite directions. The waves move through each other, with their disturbances adding together as they pass. When the two waves have the same amplitude and wavelength, they alternate between constructive and destructive interference, producing the resultant stationary waveform characteristic of standing waves.

The fixed ends of a string must be nodes because the string cannot move at those boundary points. Since nodes are locations where wave disturbance is zero and displacement is zero, the physical constraint of fixed ends naturally creates nodes. This boundary condition is essential for standing wave formation and determines which wavelengths and frequencies can exist on the string.