What is Dispersion?

Definition 3.13 High School Physics · Chapter 3 — Refraction of Light

A medium is dispersive when its refractive index depends on the wavelength of the light. In glass and water, $n$ is slightly larger for blue light (short wavelength) than for red light (long wavelength) — so blue bends more.

Examples

Example 3.14 (Two colors, one surface)

A beam of white light hits crown glass at $i_1 = 60^\circ$ . For this glass $n_{\text{red}} = 1.51$ and $n_{\text{blue}} = 1.53$ . Snell’s law, once per color:

\sin i_{\text{red}} = \frac{\sin 60^\circ}{1.51} = 0.574, \quad i_{\text{red}} = 35.0^\circ ; \qquad \sin i_{\text{blue}} = \frac{\sin 60^\circ}{1.53} = 0.566, \quad i_{\text{blue}} = 34.5^\circ .

A single surface splits white light by half a degree — invisible on a window pane, but a prism refracts twice in the same rotational sense, widens the fan, and a few meters away the colors are centimeters apart.

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Definition 5.9 University Physics — Year 1 · Chapter 5 — Signal Propagation: Travelling and Standing Waves

A medium is dispersive when the phase velocity depends on the frequency. A non-sinusoidal signal, a sum of sinusoids, then deforms as it travels, its components drifting apart. Sound in air and light in vacuum are non-dispersive; light in glass (Chapter 2) and waves on deep water are dispersive — the long swell from a distant storm arrives a day before the short chop.

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Examples

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