← all cardsTopic 11 · Wave Motion · Definitions
Phase, φ
● VERBATIM — examined word-for-wordAn angle giving a measure of the fraction of a cycle completed by an oscillating particle or wave. One full cycle = 2π rad = 360°; half cycle = π (NOT π/2). Independent of amplitude.
What this actually means
Phase answers 'where in its cycle is this particle right now?': just starting, at the crest, on the way down. Because cycles repeat, it is naturally an angle, with one full cycle equal to 2π.
Half a cycle is π, not π/2. The lecturer's warning: get this wrong and it is your maths being blamed, not your physics.
Phase carries no information about amplitude. A tiny wobble and a huge one can sit at identical phase.
The trap
Calling half a cycle π/2 — a full cycle is 2π, so half is π.
Prove it — watch it be true
- Watch the phasor: one full revolution is one complete cycle, 2π of phase
- STEP π/4 four times: half a revolution, half a cycle — π, not π/2
- Shrink the radius in your head: the angle swept is unchanged — phase ignores amplitude