Preview
Travelling Waves: Transverse, Longitudinal and Electromagnetic
IB DP Physics — Unit C: Wave Behaviour | SL & HL | C.2 Wave Model
📋 What You Will Learn
- Observe and describe how energy is transferred by waves without the bulk transfer of matter, using real-world examples
- Distinguish between transverse and longitudinal waves by identifying the relationship between oscillation direction and wave propagation direction
- Define all key wave quantities — displacement, amplitude, wavelength, period, frequency, and wave speed — and relate them through the wave equation
- Describe the nature and properties of electromagnetic waves, including the structure of the electromagnetic spectrum
- Apply the wave equation \( v = f\lambda \) to solve quantitative problems involving wave speed, frequency, and wavelength
- Analyse displacement–distance and displacement–time graphs to extract wave properties and distinguish between the two graph types
- Evaluate common misconceptions about wave motion and connect wave behaviour to other areas of the IB Physics course
🌍 Real-World Context Drop a pebble into a still pond and watch what happens: ripples spread outward in expanding circles, yet the water itself does not travel with the ripples — a floating leaf simply bobs up and down as each crest passes beneath it. The same principle governs the sound reaching your ears from a speaker, the light arriving from a distant star, and the seismic waves that travel through the Earth after an earthquake. In every case, energy is being transported from one place to another — but the medium through which the wave travels stays, on average, exactly where it started.
🔍 Guiding Question How can energy be transferred from one location to another without the net movement of matter, and what distinguishes the different types of waves that achieve this?
Physical Setup and Observation
Imagine holding one end of a long rope and giving it a sharp flick upward and then back down. A single hump — a pulse — travels along the rope toward the far end. The rope itself does not travel; each small segment of rope simply moves up and then returns to its original position as the pulse passes through it. If you repeat the flick rhythmically, a continuous train of humps and troughs travels along the rope: this is a travelling wave.
Now consider a loudspeaker cone vibrating back and forth. As the cone pushes forward, it compresses the air immediately in front of it; as it pulls back, it creates a region of lower pressure. These alternating compressions and rarefactions travel outward through the air as a sound wave. Unlike the rope, the air molecules oscillate in the same direction as the wave travels — back and forth along the direction of propagation.
Comparing the two diagrams reveals the fundamental distinction between wave types: in the rope wave, particles move perpendicular to the direction the wave travels, whereas in the sound wave, particles move parallel to the direction the wave travels. This single geometric relationship — between the direction of oscillation and the direction of propagation — is the defining criterion for classifying all mechanical waves.
Core Concepts and Definitions
Before we can write any equations, we need a precise vocabulary. Every wave, regardless of type, can be described using the same set of quantities. These quantities are defined with respect to