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First Principles of PhysicsFields & Waves · Rule 28

Fields & Waves

Rule 28Chapter IV

The medium shapes the message

Why this rule exists

A wave's speed, wavelength, and even its shape are properties of the medium it travels through, not of the source. The medium's stiffness and inertia set the propagation speed; when that speed depends on frequency, different components travel at different rates and the signal spreads. The message is carried, but the medium rewrites how it arrives.

First Principles of Physics

In practice

Before predicting how a wave arrives, characterize the medium: its wave speed, and whether that speed varies with frequency (dispersion) or direction (anisotropy). For a pulse in a dispersive medium, distinguish phase velocity from group velocity — the latter carries the energy and information. Expect attenuation, refraction at boundaries, and reshaping of the waveform over distance.

When it doesn't apply

In a non-dispersive, lossless, linear medium the waveform is preserved and this reshaping vanishes. At high amplitudes the medium responds nonlinearly, so the wave alters the medium in turn and superposition fails.

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