Q: What does A-Level Physics: 10) Wave Motion & Polarisation Guide cover? A: From basic wave vocabulary to Malus' law, this post unpacks Section III Topic 10 of the 2026 H2 Physics syllabus for IP students and parents.
TL;DR Wave Motion is not “pure theory”-it is the marks engine behind interference, optics and even Modern Physics. This guide converts the SEAB bullet-points into lesson check-lists, graph-reading drills and WA timing hacks.
1 Mechanical vs electromagnetic waves (LO a)
Mechanical waves need a medium; think slinky coils (longitudinal) or water ripples (transverse). Energy travels; the individual coils or water molecules only oscillate about equilibrium.
Electromagnetic (EM) waves are self-propagating oscillations of electric and magnetic fields in free space-no particles required.
Parent insight
A neat dinner-table demo is to compare sound (mechanical) with laser pointer light (EM). Block the speaker with a vacuum jar and the sound dies; block the laser and light still gets through.
2 Core wave vocabulary (LO b)
Symbol
Term
Quick definition
\(x\)
Displacement
How far a point is from equilibrium at an instant
\(A\)
Amplitude
Maximum displacement \( \pm A \)
\(T\)
Period
Time for one complete oscillation
\(f\)
Frequency
Oscillations per second, \( f = \dfrac{1}{T} \)
\(\phi\)
Phase
Fraction of a complete cycle, in rad or °
\(\Delta \phi\)
Phase difference
Phase gap between two points or waves
\(\lambda\)
Wavelength
Distance between consecutive points in phase
\(v\)
Speed
Distance a given phase travels per second
3 The golden relationship v=fλ(LO c,d)
Start from definitions:
v=timedistance=Tλ=fλ.
Exam drill: Convert MHz to Hz and nm to m before substitution-mist prefixes cost marks.
Four worked examples with different units feature in this explainer video.
4 Reading space- and time-base graphs (LO e)
Displacement-time graph (at one point): gradient ↔ particle velocity; peak-to-peak time = T.
Savemyexams ' diagrams are perfect for self-quiz-cover the labels and annotate crests, troughs and compressions.
5 Energy, intensity & the inverse-square law (LO f-h)
5.1 Progressive waves transfer energy, not matter
Particles only vibrate about equilibrium; net mass flow is zero.
5.2 Intensity-amplitude square law
For a progressive wave,
I∝A2.(1)
That means halving the amplitude quarters the intensity-key to sound-proofing calculations.
5.3 Inverse-square from a point source
Assuming no energy loss,
I∝r21.(2)
Origin: energy spreads over the surface area 4πr2.
Mini-drill: A torch gives 4.0W⋅m−2 at 2m. Estimate intensity at 5m.
6 Polarisation-proof that EM waves are transverse (LO i)
Definition: restriction of oscillations to one plane perpendicular to propagation.
Why only transverse? Longitudinal oscillations are parallel to propagation, so there is no “plane” to filter.
6.1 Malus' law (LO j)
For unpolarised light intensity I0 through two ideal polarisers at angle θ:
I=I0cos2θ.(3)
If θ=30∘, intensity drops to 0.75I0.
Student hack: Remember “cos squared controls colour of sunglasses” to recall Eq. (3).
7 Three WA timing rules
1 mark ≈ 1.5 min-budget your Section A MCQs.
Always copy unitsbefore numbers; it prevents prefix slips.
Quote final answers to the same sig-figs as raw data-paper 4 loves this.
8 Bridge to Practical Paper 4
Plot intensity vs distance on a log-log graph to verify gradient ≈ -2 (inverse-square).
Use phone lux meters for quick classroom demos-then verify with Eq. (2).
Comprehensive revision pack
9478 Section III, Topic 10 Syllabus outcomes at a glance
Outcome (a) - distinguish transverse and longitudinal waves, mechanical vs electromagnetic.
Outcome (b) - describe wave parameters (A,T,f,λ, phase) and relate them via v=fλ.
Outcome (c) - interpret displacement-time and displacement-distance graphs.
Outcome (d) - explain energy transport, intensity and inverse-square behaviour.
Outcome (e) - describe and quantify polarisation phenomena (Malus' law).
Concept map (in words)
Identify wave type → list properties → pick representation (t-graph or x-graph). Apply v=fλ to link frequency and wavelength. For intensity, square the amplitude and consider geometric spreading. Polarisation demonstrates transverse nature; combine with Malus' law for quantitative predictions.
\( \Delta \phi = \dfrac{2\pi \Delta x}{\lambda} \) (same \( t \)) or \( \dfrac{2\pi \Delta t}{T} \) (same \( x \))
Derivations & reasoning to master
Wave equation: show that displacement y(x,t)=Asin[2π(Tt−λx)] satisfies v=Tλ.
Intensity drop: derive the inverse-square law I∝r21 for a point source by spreading energy over a sphere area.
Malus' law: explain using projection of electric field component through successive polarisers.
Phase difference: convert between spatial and temporal separations using Δϕ=λ2πΔx=T2πΔt
Worked example 1 - graph interpretation
A displacement-distance graph shows adjacent crests 16cm apart at t = 0. A displacement-time graph at x = 0 indicates a period of 0.040s. Determine wave speed, frequency, and write the wave equation.
Unpolarised light of intensity 12W⋅m−2 passes through three polarisers. The first is aligned vertically, the second at 30∘ to the vertical, the third at 90∘ to the first. Find the transmitted intensity.
Method: after the first polariser, I=6W⋅m−2. After the second: I=6cos230∘=4.5W⋅m−2. After the third (60∘ to the second): multiply by cos260∘ ⇒ 1.1W⋅m−2.
Practical & data tasks
Use ripple tanks or simulations to visualise wavelength and frequency relationships.
Measure light intensity vs distance with a lux meter, plot log I vs log r to confirm slope ≈ -2.
Rotate polaroid filters in front of a phone camera sensor to observe Malus' law experimentally and record readings.
Common misconceptions & exam traps
Confusing particle speed with wave speed.
Forgetting that only transverse waves can be polarised.
Mixing degrees and radians when applying Malus' law.
Misreading graphs: wavelength comes from distance between crests at the same time, not just any two points.
Quick self-check quiz
If frequency doubles while speed remains constant, what happens to wavelength? - It halves.
What is the intensity transmitted through a polariser pair at 90°? - Zero.
How do you tell whether a graph is displacement-time or displacement-distance? - Look at axis labels; time axis indicates period, distance axis indicates wavelength.
Name one everyday application of polarisation. - Polarised sunglasses / LCD screens.
Why does sound not exhibit polarisation? - Sound is longitudinal in air; oscillations are parallel to propagation so no plane to filter.
Revision workflow
Recreate key definitions and equations on flashcards; test with spaced repetition.
Solve two waveform graph problems and one polarisation calculation each week.
Summarise mechanical vs EM wave characteristics in a single-page comparison chart.
Watch a resonance demo or ripple tank video and explain the physics verbally to reinforce understanding.
Practice Quiz
Test yourself on the key concepts from this guide.
Parents: book a 1-hr Wave Motion booster before WA 2; it saves future headaches in interference.
Students: memorise Eqs. (1)-(3) and test them in tomorrow's lab light-box worksheet.
Last updated 14 Jul 2025. Next review when SEAB issues the 2027 draft syllabus.
