Editing Dispersion relation (section)

=== Electromagnetic waves in vacuum ===

For [[electromagnetic wave]]s in vacuum, the angular frequency is proportional to the wavenumber:
: <math>\omega = c k.</math>

This is a ''linear'' dispersion relation, in which case the waves are said to be '''non-dispersive'''.{{sfn|Ablowitz|2011|pp=19-20}} That is, the phase velocity and the group velocity are the same:
: <math> v = \frac{\omega}{k} = \frac{d\omega}{d k} = c,</math>
and thus both are equal to the [[speed of light]] in vacuum, which is frequency-independent.