IB Physics: Transverse & Longitudinal Waves FAQ

Most marks are lost confusing displacement-time graphs with displacement-distance graphs, misreading amplitude as peak-to-peak instead of peak, and mixing up wavelength with period. Examiners also penalise vague definitions — 'oscillation perpendicular to travel' needs the word 'direction of energy transfer', not just 'motion'.

In a transverse wave, particles oscillate perpendicular to the direction of energy transfer — think light or a shaken rope. In a longitudinal wave, particles oscillate parallel to that direction, creating compressions and rarefactions — sound is the standard IB example. Both transfer energy without net particle movement.

Sound needs a medium — air molecules push forward and back along the direction the sound travels, creating pressure zones, which is longitudinal by definition. Light is an electromagnetic wave; its electric and magnetic fields oscillate perpendicular to propagation, so it's transverse and can travel through a vacuum.

Only transverse waves can be polarised because their oscillation happens in a plane perpendicular to travel, and a polarising filter can select one orientation. Longitudinal waves oscillate along the direction of travel only, so there's no perpendicular plane to restrict — polarisation is physically impossible for sound.

Read the period T off the displacement-time graph and the wavelength λ off the displacement-distance graph, then apply v = fλ where f = 1/T. Both graphs are needed together — one graph alone can't give you wave speed, a mistake examiners see constantly in Paper 1 and Paper 2.

The core equations are v = fλ (wave speed), T = 1/f (period-frequency relationship), and intensity I ∝ A² (intensity proportional to amplitude squared). All three sit in the IB Physics data booklet, so you don't memorise them — you need to know when to apply each one.

Expect graph-reading questions (extracting T and λ from displacement graphs), a definitions question testing precise vocabulary, and a calculation combining v = fλ with a real-world context like sonar or seismic waves. HL papers add standing wave diagrams and Doppler shift calculations for moving sources.

Waves underpins Topic 9 (Wave phenomena) — diffraction, interference and standing waves all rely on the transverse/longitudinal groundwork from Topic 4. It also feeds into Topic 12 (Quantum and nuclear physics), where photon behaviour is explained partly through wave properties like wavelength and frequency.

Waves is conceptually one of the more approachable Topic 4 areas — the maths is simpler than mechanics or fields — but students underperform because they rush the terminology. In my experience marking mocks, the topic itself isn't hard; careless graph-reading and vague definitions are what actually cost the marks.

Paper 1 waves questions are usually definition-based or single-step calculations, so drill precise vocabulary first, then practise reading both graph types under time pressure. Work through past-paper multiple-choice sets specifically on Topic 4 before moving to Paper 2 long-answer practice.

The core transverse/longitudinal content is identical at SL and HL — the difference shows up in Topic 9, where HL adds standing waves, resolvance and the Doppler effect with moving sources. So HL waves isn't harder at the Topic 4 stage, but it does demand more later on.

Past papers from the IB store are the gold standard for exam-style practice, but most students also need topic-specific worksheets to isolate weak spots before mixing everything into full past papers. On revisionprep.com, the Topical Worksheets and Revision Notes for DP Physics break Topic 4 down question-type by question-type.