← all cardsTopic 11 · Wave Motion · Definitions
Wavelength, λ
● VERBATIM — examined word-for-wordThe shortest distance between two points on a wave that are in phase (at the same instant) — equivalently, between successive crests/troughs or compressions/rarefactions. "Shortest" (or "successive/adjacent") is a mark-bearing word.
What this actually means
λ is a distance measured on a snapshot: from any particle to the nearest particle doing exactly the same thing at the same moment (same displacement AND same direction of motion).
'Shortest' matters because points 2λ or 3λ apart are also in phase. Without 'shortest' or 'successive', the definition picks out infinitely many distances and earns nothing.
Crest to next crest works; compression to next compression works for sound. Crest to trough is only λ/2, a classic misread.
Prove it — watch it be true
- Pick particles P and Q and snap their separation to λ
- Δφ reads zero: in phase, and this is the shortest such distance
- Snap to λ/2 instead: Δφ jumps to π — that pair is NOT in phase