Dispersion (Waves)
Definition
Why prisms make rainbows — different colors travel at slightly different speeds in glass, so white light fans out into a spectrum.
Technical definition
Dispersion is the nonlinearity of ω(k): phase velocity v_p = ω/k varies with wavelength. In normal dispersion the refractive index falls with wavelength.
Example
A prism spreads sunlight because blue light (n ≈ 1.53) bends more than red (n ≈ 1.51) — a tiny index difference with a spectacular result.
Common confusion
Dispersion is not diffraction or scattering. It is purely the speed of light depending on wavelength inside a material — in vacuum all colors travel at c.