Wave Packets
An arbitrary wave shape can be decomposed into sinusoidal waves of different wavelengths, frequencies and phases using Fourier series
a pulse requires a continuous distribution of frequencies and wavelengths: ?(k) or ?(?)
group velocity vg = ??/?k=>d?/dk
different components travel at different speeds and the wave spreads out, or disperses, with time
different wavelengths of light have different speeds in glass and spread out as well