Transverse & Longitudinal Waves

Revise Transverse & Longitudinal Waves for Physics 2 (IAL) WPH12 (AS Level) — revision notes and instant AI marking.

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Edexcel IAL Physics · Waves Topic

Transverse & Longitudinal Waves

Big idea: All waves carry energy away from a wobbling (oscillating) source — the only difference between a transverse wave and a longitudinal wave is which direction the particles wobble in, compared to which direction the wave itself travels.

Summary — What This Chapter Covers

Waves are produced by oscillating (vibrating) sources, and they transfer energy away from that source — without transferring matter.
Key wave quantities: wavelength (λ), amplitude (A), period (T), frequency (f), and wave speed (v).
f = 1/T — frequency and period are reciprocals of each other.
The wave equation, v = fλ, links speed, frequency, and wavelength for any wave.
Transverse waves: particles oscillate perpendicular (at 90°) to the direction of energy transfer. Example: light, water ripples, a guitar string.
Longitudinal waves: particles oscillate parallel to the direction of energy transfer, creating compressions and rarefactions. Example: sound.
Only transverse waves can be polarised — longitudinal waves cannot.
Waves can be represented on displacement-distance graphs (a "snapshot" of the wave) or displacement-time graphs (following one single particle over time).
Longitudinal waves can also be shown on a pressure-distance graph, which is 90° out of phase with the displacement graph.
Core Practical 4 uses an oscilloscope and signal generator to measure the speed of sound in air.

1. Properties of Waves

Every wave starts with something oscillating — vibrating back and forth around a fixed point. Think of a hand shaking one end of a rope, or a loudspeaker cone vibrating in and out. That oscillation doesn't stay put — it travels outward, away from the source, carrying energy with it but not matter. This is the single most important idea to hold onto: a wave is a travelling disturbance, not a travelling "thing."

Waves can travel through a medium (a physical substance like air, water, or a solid) or, in the case of electromagnetic waves, through a vacuum — empty space with no particles at all. Whether a wave needs a medium or not depends entirely on what type of wave it is, which we'll get to in a moment.

The five quantities you must know cold

WAVELENGTH (λ) ┌──────────┐ ╱╲ ╱╲ ╱ ╲ ╱ ╲ → crest (peak) ────╱────╲──────╱────╲────── → equilibrium (rest) line ╲ ╱ ╲ ╱ ╲ ╱ ╲ ╱ ╲ ╱ ╲╱ ╲╱ → trough ↕ AMPLITUDE (A) = distance from rest line to the very top (or bottom)

Quantity	Symbol	Unit	What it actually means
Wavelength	λ (lambda)	metres (m)	The distance from one point on a wave to the identical point on the next cycle — e.g. crest to crest, or trough to trough.
Amplitude	A	metres (m)	The maximum displacement from the rest (equilibrium) position. NOT the distance from a crest to a trough — that's double the amplitude!
Period	T	seconds (s)	The time taken for one full oscillation at a single point — i.e. one complete wave cycle to pass.
Frequency	f	hertz (Hz)	The number of complete wave cycles that pass a point every second.
Wave speed	v (or c)	m s⁻¹	How fast the wave pattern itself moves through the medium.