A-Level Physics: Wave Motion & Polarisation Guide

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)

3 The golden relationship \(v = f \lambda\) (LO c,d)

Start from definitions:

\[ v = \frac{\text{distance}}{\text{time}} = \frac{\lambda}{T} = f \lambda. \]

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 = \(T\).
• Displacement-position graph (snapshot): peak-to-peak distance = \(\lambda\).

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,

\[ I \propto A^2. \tag{1} \]

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,

\[ I \propto \frac{1}{r^2}. \tag{2} \]

Origin: energy spreads over the surface area \(4 \pi r^2\).

Mini-drill: A torch gives \(4.0 \space \text{W/m2}\) at 2 m. Estimate intensity at 5 m.

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 \(I_0\) through two ideal polarisers at angle \(\theta\):

\[ I = I_0 \cos^2 \theta. \tag{3} \]

If \(\theta = \pu{30 ^\circ}\), intensity drops to \(0.75I_0\).

Student hack: Remember “cos squared controls colour of sunglasses” to recall Eq. (3).

7 Three WA timing rules

1. 1 mark ≈ 1.5 min—budget your Section A MCQs.
2. Always copy units before numbers; it prevents prefix slips.
3. 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).

9 Further reading

• SEAB H2 Physics 9478 syllabus (2026)
• Digestible Notes — Progressive Waves
• OpenStax University Physics — Polarisation
• Save My Exams — Graphical Wave Representations
• HyperPhysics — Inverse-Square Law (Sound)

10 Call-to-action

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.