If you have ever felt sound in your chest at a concert, you have already met a longitudinal wave. Nothing dramatic moves from the speaker to you, no air parcel travels across the room like a tiny delivery drone. Yet the energy arrives anyway, right on time. That quiet contradiction sits at the heart of IB Physics, and it is exactly why longitudinal waves show up so often in exam questions.
Longitudinal wave quick checklist (IB Physics)
Use this before you practise questions:
-
State the definition: oscillations are parallel to direction of travel.
-
Use the language of compressions and rarefactions.
-
Identify wavelength correctly: compression-to-compression (or rarefaction-to-rarefaction).
-
Apply the wave equation: v = fλ.
-
Contrast with transverse waves (light is not longitudinal).
-
Drill exam-style prompts in the C.2 Wave model questionbank.
What a longitudinal wave is (and how IB Physics marks it)
A longitudinal wave is a travelling disturbance where particles of the medium oscillate back and forth in the same direction the wave propagates. In IB Physics, that “same direction” wording matters because it is often the single mark in a definition question.
Think of a slinky: you push and pull coils along the length of the spring, and the disturbance moves down the line. The coils do not migrate to the far end overall; they oscillate about equilibrium. That is the deeper idea behind many IB Physics marking points: energy transfer without net matter transfer.
To lock in exam language, it helps to keep the official vocabulary close, like the IB Physics Key Definitions.
Compressions, rarefactions, and wavelength (λ)
Longitudinal waves are easiest to describe in terms of pressure/density changes:
-
Compressions: particles are closer together (higher pressure/density).
-
Rarefactions: particles are further apart (lower pressure/density).
In IB Physics, wavelength in a longitudinal wave is the distance between two consecutive compressions, or two consecutive rarefactions. Many students try to measure “one compression plus one rarefaction” and accidentally halve (or double) λ.
If you want a clean conceptual recap of how waves propagate through a medium, the notes on C.2.2 Nature of wave propagation are a strong anchor.
Longitudinal vs transverse waves in IB Physics
Examiners like comparison questions because they reveal whether you are thinking in diagrams or definitions.
Core contrast
-
Longitudinal: oscillations parallel to propagation (sound, seismic P-waves).
-
Transverse: oscillations perpendicular to propagation (light, waves on a string).
A reliable memory hook for IB Physics: mechanical waves can be longitudinal or transverse, but electromagnetic waves (like light) are transverse and can travel in vacuum.
For the broader wave toolkit (terms like amplitude, frequency, and phase), revise alongside 4.3 Wave characteristics notes.
The one equation you must deploy: v = fλ
Longitudinal waves obey the same wave relationship as any travelling wave:
v = fλ
Where:
-
v is wave speed (m s⁻¹)
-
f is frequency (Hz)
-
λ is wavelength (m)
In IB Physics, many “sound in air” questions are disguised unit checks. If you keep v in m s⁻¹, f in Hz, and λ in m, you prevent the classic cm-to-m slip that costs quiet marks.
To practise this in context, use the broader Oscillations and Waves hub and then jump into targeted practice.
Where longitudinal waves appear in exam questions
Longitudinal waves turn up in predictable storylines:
-
Sound and resonance: tubes/air columns, using f and λ, and interpreting nodes/antinodes (often linked conceptually to standing waves notes).
-
Seismic waves: identifying P-waves as longitudinal, S-waves as transverse.
-
Doppler effect (sound): explaining frequency change with motion.
When you are ready to simulate exam pressure without guessing what to revise, RevisionDojo’s IB Physics Predicted Papers make the patterns feel familiar.
Mistakes IB Physics students keep repeating
-
Saying “light is longitudinal” because it “travels.” In IB Physics, light is electromagnetic and transverse.
-
Describing compressions/rarefactions but forgetting to mention parallel oscillations.
-
Measuring λ incorrectly on a longitudinal diagram.
-
Implying sound travels in vacuum (it needs a medium).
Mini routine: how to revise longitudinal waves efficiently
A calm 20-minute loop that works:
-
Read the key explanation in C.2 Wave model notes.
-
Do 10 questions from the Oscillations and Waves questionbank and check where your definitions are vague.
-
Make 6 flashcards: definition, compressions, rarefactions, wavelength, v=fλ, transverse comparison. Then drill them (RevisionDojo Flashcards pair well with this).
-
Ask RevisionDojo AI Chat to mark your definition and rewrite it in examiner language.
Closing: turn the definition into marks
A longitudinal wave is simple on paper: parallel oscillations, compressions and rarefactions, and v = fλ. The exam difficulty comes from being slightly careless with language, diagrams, or wavelength. Build the habit of writing the definition cleanly, then prove it with one labelled idea (compression/rarefaction) and one equation step.
If you want this topic to feel automatic in IB Physics, RevisionDojo is built for that moment: Study Notes to clarify, Questionbank to pressure-test, Flashcards for recall, AI Chat and Grading tools for feedback, Predicted Papers and Mock Exams for timing, plus a Coursework Library and Tutors when you need a human check-in. Start with the waves resources, and make longitudinal waves one of the easiest marks you collect.
