Physics · Waves. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A stationary wave forms when two waves of the same frequency and amplitude travel in opposite directions and superpose, for example a wave and its reflection. The pattern does not move along and no energy is carried along it.
Points that never move are nodes. Points that vibrate with the greatest amplitude are antinodes. Adjacent nodes are half a wavelength apart, and a node is a quarter of a wavelength from the next antinode.
Only certain frequencies fit. A string fixed at both ends has a node at each end. In an air column there is a node at a closed end and an antinode at an open end. These conditions decide which harmonics are possible.
Giving a closed pipe the frequencies 2f1, 4f1. A pipe closed at one end has only odd harmonics, so after f1 the next frequency is 3f1.
Answer: 50 Hz.
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