C.4.2—Nodes and antinodes

Identify the positions

A node is a fixed position where the displacement is always zero. An antinode is a fixed position where the amplitude is greatest. Adjacent nodes or adjacent antinodes are separated by $\lambda/2$; a node and its nearest antinode are separated by $\lambda/4$.

Read relative amplitude

Every point between two adjacent nodes oscillates at the same frequency, but its amplitude depends on position: zero at a node, maximum at an antinode, and intermediate elsewhere. The standing-wave envelope therefore describes amplitude, not a travelling displacement profile at one instant.

Read phase

Points in the same segment between adjacent nodes oscillate in phase. Points in neighbouring segments oscillate in antiphase, with phase difference $\pi$ (180°). At a node the phase is not useful to assign because the displacement amplitude is zero.

Common trap

Do not infer phase only from distance. First locate the nodes: crossing one node changes the phase by $\pi$; staying within the same node-to-node segment leaves the phase difference zero.