Wave Behaviour
IB Physics Guide (2023 first exams) – Theme C: Wave behaviour, C.1–C.3 (SL content only; HL extensions flagged)
Everything in this topic is one wave doing different things depending on what it meets. Learn properly and you can describe any wave; learn and you understand every oscillator that generates a wave in the first place; learn what happens at a boundary (reflect, refract, diffract, interfere) and you can explain fibre optics, rainbows, X-ray diffraction and redshift with the same four ideas. Don't treat SHM and waves as separate chapters — the source of every periodic wave is something oscillating in SHM.
Overview: How the Topic Fits Together
What this topic is really about
A wave transfers energy (and momentum) through a medium, or through a vacuum for electromagnetic waves, without any net transfer of matter — the water molecules in an ocean wave bob up and down, they don't travel to shore. Once you accept that, three ideas cover the whole syllabus.
- Wave model gives you the vocabulary and quantities — wavelength , frequency , period , amplitude , speed — needed to describe any wave mathematically, plus what happens when identical waves overlap (superposition, standing waves).
- Wave phenomena are the behaviours any wave shows at a boundary, obstacle, or with relative motion: reflection, refraction, diffraction, interference, polarization, Doppler shift.
- Simple harmonic motion is the oscillation pattern of the source that creates a periodic wave in the first place — a loudspeaker cone, a vibrating string, an electron in a radio antenna all move with SHM.
The shape of the chapter
Command terms as they bite in this topic
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Define | give the precise IB wording for a quantity or law | AO1 | 'wavelength is the distance between two successive points in phase' — paraphrasing loses the mark even if the meaning is right |
| Describe | give a detailed, feature-by-feature account | AO2 | describing a standing wave needs BOTH node positions AND antinode positions stated, not just 'it looks like a wave pattern' |
| Explain | give the underlying mechanism/reason, not a restatement | AO3 | 'the ray bends because the wave slows down entering the denser medium' — not just 'it refracts' |
| Sketch | simple diagram with correct shape/relative features | AO2 | phase relationships (e.g. a-t inverted relative to x-t) must be shown even without axis values |
| Determine / Calculate | numerical answer with working shown | AO2/AO3 | a correct bald final answer with no working and wrong/missing units drops the final mark |
| Compare | explicit similarities AND differences | AO3 | listing two separate descriptions without connecting words ('whereas', 'unlike') scores fewer marks than a direct comparison |
Key point
Overview