Unit Overview

Theme C: Wave behaviour

IB Physics HL· 5 min read 📊 15-17% of overall exam

1. Unit at a Glance

We start with the foundational oscillatory motion of simple harmonic motion (SHM), the repeating motion that acts as the building block for all wave phenomena. From SHM, we move to the core properties of travelling waves, before exploring fundamental wave behaviours like reflection, refraction, dispersion and polarisation.

The unit progresses to standing wave formation and resonance, then covers AHL extensions building on core concepts: the Doppler effect, multi-source interference, diffraction, optical resolution, and thin film interference. Topics build progressively, so we recommend working through sub-topics in order.

This unit includes the following core and AHL sub-topics:

C.1 Simple harmonic motion

Learn the defining characteristics, energy changes and mathematical description of SHM.

★★⏱ 10 min

C.2 Travelling waves

Explore wave types, properties, and the relationship between speed, frequency and wavelength.

★★⏱ 8 min

C.3 Wave phenomena

Cover core wave behaviours including reflection, refraction, dispersion and polarisation.

★★★⏱ 10 min

C.4 Standing waves and resonance

Analyze standing wave formation via superposition and resonance conditions for different systems.

★★★⏱ 9 min

C.5 Doppler effect (AHL)

Derive and apply Doppler effect formulas for moving sources and observers for sound and light.

★★★★⏱ 10 min

C.6 Interference (AHL)

Study conditions for constructive and destructive interference from multiple coherent sources.

★★★★⏱ 11 min

C.7 Diffraction and resolution (AHL)

Analyze single-slit diffraction and apply the Rayleigh criterion for optical resolution.

★★★★⏱ 10 min

C.8 Thin film interference (AHL)

Calculate interference conditions for thin films, accounting for phase changes upon reflection.

★★★★★⏱ 12 min

2. Common Pitfalls

Wrong move:

Forgetting to account for 180° phase changes when calculating thin film interference path differences.

Why:

A phase change adds an extra $λ /2$ effective path difference that reverses constructive/destructive conditions.

Correct move:

Always check each reflection to see if a phase change occurs from a higher refractive index medium.

Wrong move:

Confusing Doppler effect equations for moving sources vs moving observers.

Why:

Frequency shift formulas differ depending on which object is moving relative to the wave medium.

Correct move:

Derive from first principles if unsure, remember $f^{'}$ increases when source/observer move towards each other.

Wrong move:

Mixing up wavelength values in thin film interference calculations.

Why:

Wavelength in a medium of refractive index $n$ is shorter than wavelength in vacuum.

Correct move:

Always use $λ_{n} = \frac{λ}{n}$ for the wavelength inside the thin film.

3. Quick Reference Cheatsheet

Concept	Key Formula/Rule
SHM defining equation	$a = - ω^{2} x$
Universal wave speed relation	$v = f λ$
Brewster's law for polarisation	$n = tan θ_{p}$
First harmonic (fixed string)	$f_{1} = \frac{v}{2 L}$
Doppler effect (moving source)	$f^{'} = f \frac{v}{v \mp u _{s}}$
Double-slit constructive interference	Path difference = $nλ$ , $n = 0, 1, 2...$
Rayleigh resolution criterion	$θ = 1.22 \frac{λ}{b}$
Thin film (1 phase change, constructive)	$2 n t = (m + \frac{1}{2}) λ$

What's Next

Start your exploration of this unit with the first sub-topic, C.1 Simple harmonic motion, the foundational oscillatory motion that underpins all wave behaviour. Work through each sub-topic in order, as concepts build progressively from core fundamentals to advanced AHL applications. Once you complete all sub-topics in this unit, move on to the next IB Physics HL unit covering circular motion and gravitation.