IB Physics waves are best understood as a small set of connected models: oscillation, wave motion, superposition, interference, standing waves, resonance and the Doppler effect. For exams, you must do more than recall formulas. You need to interpret graphs and diagrams, identify the correct model, justify relationships and present calculations clearly.
In the current course, first assessed in 2025, waves appear under Theme C: Wave behaviour. The theme has 17 recommended teaching hours at SL and 29 at HL, with additional HL material in parts of C.1, C.3 and C.5. This guide explains the central ideas, the language used in questions and the methods that convert understanding into marks.
Where waves appear in the current IB Physics course
The official IB Physics subject brief organizes the course conceptually rather than treating every chapter as isolated content. Theme C contains five connected areas:
| Syllabus area | Central question |
|---|---|
| C.1 Simple harmonic motion | How do restoring forces produce periodic motion? |
| C.2 Wave model | How do disturbances transfer energy? |
| C.3 Wave phenomena | What happens when waves reflect, refract, diffract and overlap? |
| C.4 Standing waves and resonance | How do boundary conditions produce natural frequencies? |
| C.5 Doppler effect | How does relative motion change observed frequency or wavelength? |
The current external assessment consists of Paper 1A multiple-choice questions, Paper 1B data-based questions and Paper 2 short-answer and extended-response questions. The subject brief assigns 36% to Paper 1 collectively and 44% to Paper 2, while the scientific investigation contributes the remaining 20%.
Wave questions can therefore appear as rapid conceptual decisions, graph-based data analysis or longer calculations. The IB Physics Topic C questionbank is useful because it separates these different question styles.
The wave model and equations examiners expect
A wave is a propagating disturbance that transfers energy and information without producing a net transfer of matter. Mechanical waves require a medium, whereas electromagnetic waves can propagate through a vacuum.
The foundational relationship is:
v = fλ
Here, v is wave speed, f is frequency and λ is wavelength. Also remember f = 1/T, where T is the period.
A frequent mistake is assuming that frequency changes when a wave enters another medium. The source fixes the frequency, so at a boundary the frequency remains constant while speed and wavelength may change. If speed decreases, wavelength decreases in the same proportion.
Transverse and longitudinal waves
In a transverse wave, oscillations are perpendicular to the direction of energy propagation. Electromagnetic waves and waves on a stretched string are standard examples.
In a longitudinal wave, oscillations are parallel to the direction of propagation. Sound in air consists of moving compressions and rarefactions, not a sinusoidal path followed by individual air particles.
When asked to “distinguish” these wave types, state the orientation of oscillation relative to propagation. Merely listing examples may not earn the defining mark.
Reading wave graphs correctly
A displacement-distance graph shows the shape of the wave at one instant. Measure wavelength horizontally between adjacent points in phase, such as crest to crest.
A displacement-time graph shows the motion of one position in the medium. Measure period horizontally, then use f = 1/T. Examiners commonly test whether students can distinguish these two graph types before performing a calculation.
Phase difference can be written as:
phase difference = 2πΔx/λ = 2πΔt/T
Points separated by one wavelength are in phase. Points separated by half a wavelength are in antiphase.
Simple harmonic motion as the source of waves
Simple harmonic motion, or SHM, occurs when acceleration is proportional to displacement from equilibrium and directed toward equilibrium:
a = -ω²x
The negative sign matters because it represents the restoring direction. At maximum displacement, speed is zero and acceleration magnitude is maximum. At equilibrium, acceleration is zero and speed is maximum.
IB questions often present an unfamiliar oscillator and ask you to show that it performs SHM. The correct method is to derive an expression of the form a = -constant × x. Do not simply state that the motion is periodic, because periodic motion is not necessarily simple harmonic.
For a mass-spring system, T = 2π√(m/k). For a simple pendulum undergoing small-angle oscillations, T = 2π√(L/g). HL students should also be prepared to connect displacement, velocity, acceleration, phase and energy mathematically.
Reflection, refraction and diffraction
During reflection, the angle of incidence equals the angle of reflection, with both angles measured from the normal. A pulse reflected from a fixed end is inverted, while reflection from a free end is not inverted.
Refraction occurs because wave speed changes at a boundary. For light:
n = c/v
n₁ sin θ₁ = n₂ sin θ₂
A ray entering a slower optical medium bends toward the normal. Frequency remains unchanged, so the reduced speed produces a shorter wavelength.
Total internal reflection requires travel from higher refractive index to lower refractive index and an incidence angle greater than the critical angle. At the critical angle, the refracted ray travels along the boundary, giving sin c = n₂/n₁ for n₁ > n₂.
Diffraction is the spreading of a wave after passing through an aperture or around an obstacle. It is most significant when the aperture width is comparable to or smaller than the wavelength. Avoid saying that diffraction occurs only when the gap equals one wavelength; diffraction always occurs, but its extent changes.
Superposition and interference
The principle of superposition states that when waves overlap, the resultant displacement is the vector sum of their individual displacements. Interference is the observable pattern produced by this superposition.
For coherent waves with a constant phase relationship:
- Constructive interference occurs when path difference is mλ.
- Destructive interference occurs when path difference is (m + 1/2)λ.
- Intensity is proportional to the square of amplitude, so doubling amplitude produces four times the intensity.
In double-slit problems, maxima satisfy d sin θ = mλ. For a distant screen and small angles, fringe spacing is approximately s = λD/d, where D is screen distance and d is slit separation.
If asked what happens when slit separation increases, state that fringe spacing decreases because s is inversely proportional to d. The explanation, not just the direction of change, is often required. HL students should pay particular attention to quantitative diffraction and interference patterns covered in the C.3 Wave phenomena notes and C.3 exam-style questionbank.
Standing waves, harmonics and resonance
A standing wave forms when two identical waves of the same frequency and amplitude travel in opposite directions and superpose. A node has zero amplitude, while an antinode has maximum amplitude.
Adjacent nodes are separated by λ/2, as are adjacent antinodes. A node and its nearest antinode are separated by λ/4. All points between adjacent nodes oscillate in phase, while points in neighbouring loops oscillate in antiphase.
Boundary conditions determine the allowed frequencies:
| System | Fundamental condition | Allowed frequencies |
|---|---|---|
| String fixed at both ends | L = λ/2 | fₙ = nv/(2L) |
| Pipe open at both ends | L = λ/2 | fₙ = nv/(2L) |
| Pipe closed at one end | L = λ/4 | fₙ = nv/(4L), for odd n only |
For air columns, an open end is a displacement antinode and a closed end is a displacement node. Be careful if a diagram instead represents pressure variation, because pressure nodes and displacement nodes occur at opposite locations.
Resonance occurs when a driving frequency matches or is close to a natural frequency, producing maximum amplitude and efficient energy transfer. Damping lowers the resonance peak and broadens the range of frequencies over which the system responds. Critical damping returns a disturbed system to equilibrium as quickly as possible without oscillation, while heavy damping produces a slower non-oscillatory return.
The C.4 standing waves and resonance resources provide targeted practice on pipes, strings, harmonics and resonance curves.
The Doppler effect
The Doppler effect is the change in observed frequency caused by relative motion between a source and an observer. Approach produces a higher observed frequency; separation produces a lower observed frequency.
For sound, students should first identify whether the source, observer or both are moving. A general expression is:
f observed = f source × (v ± v observer)/(v ∓ v source)
Choose signs by physical reasoning rather than memorizing a sign pattern: motion that brings source and observer together must increase the observed frequency. The speed v is the wave speed relative to the medium.
For electromagnetic waves at relative speeds much smaller than the speed of light, the magnitudes satisfy approximately |Δf|/f ≈ |Δλ|/λ ≈ u/c. Frequency and wavelength shifts have opposite signs: redshift means increased wavelength and reduced frequency, while blueshift means reduced wavelength and increased frequency.
How IB examiners phrase wave questions
Command terms determine the required response:
- State: give a concise answer without derivation.
- Determine: obtain the answer from supplied information, often including working.
- Calculate: substitute correctly, retain units and give a sensible number of significant figures.
- Explain: provide a linked physical cause and consequence.
- Show that: begin from known information and demonstrate the stated result without circular reasoning.
- Sketch: show the correct shape, intercepts, labels and important features rather than artistic detail.
For calculations, write the relevant equation before substitution, keep unrounded values during working and include the final unit. For explanations, use a chain such as: speed decreases, frequency remains fixed, therefore wavelength decreases and the ray changes direction.
After learning a method, watch it applied to complete questions. The RevisionDojo IB Physics hub and its Physics video library provide worked instruction, while the platform's per-question worked video solutions show how equations, diagrams and markscheme language fit together. Use legally obtained past papers from your school, attempt each question first, and then compare your method with the worked solution rather than passively watching.
Common mistakes that lose marks
- Confusing amplitude with peak-to-peak displacement.
- Reading wavelength from a displacement-time graph.
- Changing frequency when a wave crosses a stationary boundary.
- Measuring optical angles from the surface rather than the normal.
- Treating a standing-wave diagram as several wavelengths without checking node spacing.
- Saying resonance gives “infinite amplitude” rather than a large, damping-limited amplitude.
- Applying a Doppler formula before identifying which object moves relative to the medium.
- Giving a numerical answer without units or sufficient working.
A productive revision sequence is concept review, short retrieval and exam practice. The Theme C flashcards can reinforce definitions, but calculations must be practised through the full IB Physics questionbank. Jojo AI can then help identify whether an error came from physics, algebra or interpretation.
Conclusion
IB Physics waves centre on a manageable set of connected ideas: oscillations generate waves, wave speed links frequency and wavelength, superposition produces interference, boundaries create standing waves, and relative motion produces Doppler shifts. Exam success depends on recognizing which model applies and communicating the reasoning with equations, diagrams, units and precise terminology.
Use RevisionDojo Study Notes and Flashcards to secure the concepts, then move quickly to the Questionbank and per-question worked video solutions. Timed IB Physics predicted papers are most useful after you can solve individual wave questions consistently.
Sources and referenced URLs
- Official IB Diploma Programme Physics subject brief, first assessment 2025
- RevisionDojo IB Physics resources
- RevisionDojo Physics video library
- IB Physics Topic C Wave Behaviour questionbank
- IB Physics C.3 Wave Phenomena notes
- IB Physics C.3 Wave Phenomena questionbank
- IB Physics C.4 Standing Waves and Resonance resources
- IB Physics Theme C flashcards
- RevisionDojo IB Physics questionbank
- RevisionDojo IB Physics predicted papers