Interference & Stationary Waves

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

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Edexcel IAL Physics — Waves

Interference & Stationary Waves

Superposition Path Difference Nodes & Antinodes Wave Speed on a String Core Practical 5

The Big Idea: When two waves of the same frequency overlap, they add together point-by-point — and depending on exactly how "in step" they are when they meet, they can either boost each other into a bigger wave (constructive) or cancel each other out (destructive). When this happens continuously between a wave and its own reflection, you get a special pattern that looks like it's standing still — a stationary wave.

Quick Summary

Superposition is what happens when two or more waves meet — their displacements add together to give one resultant wave.
Constructive interference = waves add up to a bigger amplitude. Destructive interference = waves cancel to a smaller (or zero) amplitude.
Interference is only visible if the sources are coherent — same frequency, constant phase difference.
Phase difference is about how "out of step" two wave cycles are (measured in degrees/radians). Path difference is about how much extra distance one wave has travelled (measured in wavelengths).
Constructive interference happens when path difference = nλ (whole number of wavelengths). Destructive happens when path difference = (n + ½)λ.
A stationary wave forms when two waves of the same frequency and amplitude travel in opposite directions and superpose — usually a wave and its own reflection.
Stationary waves have fixed nodes (zero displacement, always) and antinodes (maximum displacement, oscillating up and down).
Wave speed on a stretched string: v = √(T/μ). Fundamental frequency: f₀ = (1/2L)√(T/μ).
Core Practical 5 investigates how the frequency of the first harmonic depends on string length, tension, or mass per unit length.

1. Interference & Superposition of Waves

Picture two ripples on a pond heading toward each other. What happens the instant they cross paths? They don't bounce off each other or get destroyed — instead, at every point where they overlap, the water surface height is simply the sum of what each wave would have done on its own. Once they've fully passed through each other, both ripples carry on completely unchanged, as if the meeting never happened.

This "just add the displacements together" rule is called the principle of superposition, and the process itself is called interference. It applies to any type of wave — water, sound, light, waves on a string — as long as they're travelling through the same medium at the same time.

Principle of Superposition

Resultant displacement = Sum of individual displacements

In plain English: at any point where waves overlap, just add up (with correct sign) what each wave is doing there.

Constructive vs Destructive Interference

There are two extreme outcomes of superposition, depending on how the waves line up:

2. Phase & Path Difference

These two terms sound alike, and students mix them up constantly on exams — but they describe two genuinely different things. Let's separate them clearly.