Transverse and longitudinal waves
Transverse wave: oscillation is perpendicular to the direction of travel; examples: all EM waves and water surface waves. Longitudinal wave: oscillation is parallel to travel, with compressions (particles closer, higher pressure) and rarefactions (further apart, lower pressure); example: sound. Both transfer energy without permanently moving matter. State the direction relative to travel — "up and down" alone scores zero.
Five wave quantities; v = fλ and f = 1/T
Amplitude : maximum displacement from the equilibrium position; m — NOT crest-to-trough (that is ). Wavelength : distance between successive in-phase points (crest to crest); m. Frequency : complete waves per second; Hz. Period : time for one cycle; s. Wavefront: a line joining points in phase. Formulae (m/s) and . All EM waves travel at m/s in a vacuum.
Reflection, refraction, Doppler, diffraction
Reflection: angle of incidence = angle of reflection, from the normal. Refraction: entering a new medium, speed and wavelength change but frequency is unchanged; into a slower medium it bends towards the normal (E). Doppler effect: a source moving towards you gives shorter wavelength, higher frequency; away, longer, lower — speed unchanged. (E) Diffraction: waves spread through a gap, most when the gap ≈ the wavelength.
Drawn from real examiner reports.
Say "perpendicular/parallel to travel"
Wave-type definitions need TWO elements: the direction of oscillation AND that it is relative to the direction of travel. "Vibrates up and down" scores zero. Correct: transverse = perpendicular to travel; longitudinal = parallel to travel, with compressions and rarefactions. Sound is always longitudinal; all EM waves are transverse — never swap them.
November 2023 Paper 0625/11 Q17 — examiner reported that all four options were approximately equally popular, confirming widespread failure to recall the correct definition of transverse vs longitudinal. June 2024 Paper 0625/33 Q6(a) — "some confusion between the motion of the particles in a longitudinal wave and a transverse wave was seen; a number of candidates did not give an answer."
Amplitude ≠ crest-to-trough; convert cm to m
Two mechanical traps: (1) Amplitude is equilibrium-to-crest, not crest-to-trough (that is ) — reading crest-to-trough doubles the answer. (2) Convert wavelength cm → m before : 6.0 cm = 0.060 m; using 6.0 gives a frequency 100× too small. Convert period ms → s before .
June 2024 Paper 0625/12 Q16 — "weaker candidates struggled to apply their knowledge of wavelength and amplitude to the diagram, choosing mainly options A or B." June 2024 Paper 0625/22 Q22 — "the most common errors made by weaker candidates included using the wrong value for the wavelength and forgetting to convert the units into metres."
Refraction changes speed/λ, not frequency
When a wave refracts at a boundary its speed and wavelength change (in the same ratio) and its direction changes, but its frequency stays the same. Cambridge P3 specifically asks "what happens to the frequency?" — the answer is always unchanged. Writing "frequency changes" scores zero.
Frequency and period are reciprocals
Frequency (Hz) is waves per second; period (s) is seconds per wave — they are reciprocals, . A period of 2.0 ms is s, giving Hz (not 2 Hz or 0.002 Hz). Always convert ms to s before using .
(E) Sonar: halve the total distance
(E) In echo sounding the pulse travels to the surface and back, so gives the total path — the depth is half of it. For s in sea water ( m/s): total m, depth m. Forgetting to halve (giving 480 m) is the most common error.
Write the formula in symbols first
Method marks (C) go for writing the formula ( or ) before substituting; the answer mark (A) needs the value with a unit. A bare number forfeits the method mark. On "show that" the printed answer means all marks depend on shown working.
Read wave graphs: axis and equilibrium line
On a displacement graph, read amplitude from the equilibrium line to a crest (vertical) and wavelength crest-to-crest (horizontal). Check the x-axis: distance / m → read wavelength; time / s → read period. The y-axis is displacement / m.
(E) Diffracted wavefronts keep the same λ
(E) When drawing diffraction through a gap, show the wavefronts spreading out (semicircular for a narrow gap) with the same wavelength as before the gap — do not make them converge or shrink. Diffraction is greatest when the gap width is about equal to the wavelength.
| Quantity | Symbol | Formula | Unit |
|---|---|---|---|
| Wave speed | m s | ||
| Frequency | Hz | ||
| Period | s | ||
| Wavelength | m | ||
| Amplitude | maximum displacement from equilibrium | m |
Where = wave speed (m s), = frequency (Hz), = wavelength (m), = period (s).
Speed of all EM waves in a vacuum (and approximately in air): m s
Transverse wave: oscillation is perpendicular to the direction of wave travel and energy transfer. Examples: all EM waves (light, radio waves, microwaves, infrared (IR), ultraviolet (UV), X-rays, gamma rays) and water surface waves.
Longitudinal wave: oscillation is parallel to the direction of wave travel, producing alternating compressions (high pressure) and rarefactions (low pressure). Example: sound waves in any medium.
Both wave types transfer energy without permanently transferring the matter through which they travel.
Define the period of a wave.
A ripple tank generates water waves with a frequency of 5.0 Hz. A photograph of the ripple tank shows crests that are 3.8 cm apart.
Calculate the speed of the water waves.
