The principle of superposition and the interference of two waves. Reflection at a fixed and at a free end. Standing waves and the normal modes of a string, and of pipes closed at one end or open at both.
3 lessons, about 1 h 30 min3 simulations12 problems6 practice questionsPython in every lesson
When two waves meet, the displacement at each point is the sum of their displacements. This is the principle of superposition. Two waves of the same frequency with a phase difference add to a wave of amplitude , so they can help each other or cancel.
A wave reflected at a fixed end comes back inverted, and at a free end it comes back upright. The incident and reflected waves add to a standing wave , with nodes apart. A string tied at both ends vibrates in normal modes .
Air in a pipe holds standing waves too. A closed end is a node and an open end an antinode. A pipe closed at one end has only odd harmonics, . A pipe open at both ends has every harmonic, .
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