Chapter revision
Revision summary
Key ideas, equations and common mistakes. Open any topic below for the full explanation.
- Wave motion
- A travelling disturbance transfers energy without matter travelling with it. Compare material vibration with propagation: perpendicular for transverse, parallel for longitudinal.
- Frequency and period
- f = 1/T. Frequency is complete cycles per second, in Hz. Period is time per complete cycle, in s. A complete cycle contains two equilibrium crossings, one in each direction.
- Wave speed
- v = fλ = λ/T. Use f in Hz and λ in m for v in m/s. Wave speed is the pattern's propagation speed.
Choose the graph for the quantity
| Representation | What it gives directly |
|---|---|
| Displacement-distance snapshot | Different points at one instant. Read wavelength from equivalent adjacent points and amplitude from equilibrium to maximum displacement. |
| Displacement-time trace | One point at different instants. Read period from a complete cycle and amplitude from maximum displacement; use f = 1/T. |
| Ripple-tank wavefronts | Each front joins points at the same stage, such as one crest. Travel is perpendicular to the local front in the uniform region shown. |
Keep the physical explanation
- A marked part of a rope or spring oscillates locally; a crest or compression travels.
- A wavelength is one full spatial cycle; compression to neighbouring rarefaction is half of one. Amplitude is half the crest-to-trough displacement.
- Six consecutive crests span five wavelengths. Timing several cycles is different from timing a pulse's travel between two positions.
Units and measurement reminders
1 cm = 0.01 m; 1 ms = 0.001 s; 1 kHz = 1000 Hz. Match the units before substitution. Use a calibrated scale, count full wavelength intervals and complete cycles, keep propagation conditions steady, and choose timing resolution that can resolve the event. Repetition does not correct an incorrect scale.
Back to wave motionPractise an exam question
Write your answer first, then tick the mark points it makes.
Exam-style question
Water waves travel from deep water into shallower water near a beach. State and explain what happens to their speed, frequency and wavelength. [3]
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