Unit Overview

Wave behaviour

IB Physics SL· 5 min read 📊 18-20% of overall IB Physics SL exam

1. Unit at a Glance

We start with simple harmonic motion (SHM), the simplest form of repeating oscillatory motion, before expanding to generalized travelling wave properties. The unit progresses from describing individual waves to exploring how multiple waves interact with each other via superposition.

This sequence builds from foundational definitions to complex applied problems that commonly appear on IB exams. All sub-topics build on concepts introduced earlier in the unit.

The core sub-topics in this unit are:

Simple harmonic motion

Learn the definition, kinematics, and energy of SHM systems including mass-springs and pendulums.

★★⏱ 10 min

Wave characteristics

Distinguish between transverse and longitudinal waves and learn core wave parameters.

★★⏱ 8 min

Wave interference

Apply superposition to explain constructive and destructive interference in common experimental setups.

★★★⏱ 9 min

Standing waves

Analyze standing wave patterns on strings and air columns and solve harmonic frequency problems.

★★★⏱ 10 min

2. Common Pitfalls

Wrong move:

Confusing SHM acceleration direction, assuming acceleration always follows motion.

Why:

SHM acceleration is always directed toward equilibrium, opposite displacement.

Correct move:

Remember $a = - ω^{2} x$ : the negative sign confirms acceleration opposes displacement from equilibrium.

Wrong move:

Mixing up boundary conditions and harmonics for open vs closed air columns.

Why:

Closed ends form nodes and open ends form antinodes, which changes the allowed harmonic sequence.

Correct move:

Start by identifying boundary conditions, then derive allowed wavelengths/frequencies from there.

3. Quick Reference Cheatsheet

Concept / Formula	Key Description
$a = - ω^{2} x$	Defining relationship for simple harmonic motion
$v = f λ$	Universal wave speed relation for all travelling waves
Constructive interference path difference: $nλ$ , $n = 0, 1, 2...$	Condition for constructive superposition of two coherent waves
Destructive interference path difference: $(n + \frac{1}{2}) λ$ , $n = 0, 1, 2...$	Condition for destructive superposition of two coherent waves
Fixed string (both ends): $f_{n} = \frac{n v}{2 L}$ , $n = 1, 2, 3...$	Harmonic frequencies for standing waves on a fixed string
Closed tube (one open end): $f_{n} = \frac{n v}{4 L}$ , $n = 1, 3, 5...$	Harmonic frequencies for standing sound waves in a closed tube

What's Next

Begin your study of this unit with the first sub-topic on simple harmonic motion, which builds the foundational oscillatory concepts you will need for all later wave topics. Work through the sub-topics in order, as each one depends on knowledge from the previous. Once you complete all topics in this unit, you will move on to the next unit on thermal physics.