A-Level Physics — 10) Wave Motion & Polarisation (IP-Friendly Guide)
Download printable cheat-sheet (CC-BY 4.0)14 Jul 2025, 00:00 Z
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 \((+A \space \text{or}\space-A)\) |
\(T\) | Period | Time for one complete oscillation |
\(f\) | Frequency | Oscillations per second, \(f = 1/T\) |
\(\phi\) | 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 \(v = f \lambda\) (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 mark ≈ 1.5 min—budget your Section A MCQs.
- Always copy units before 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).
9 Further reading
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.