The name "dispersion relation" originally comes from optics. It is possible to make the effective speed of light dependent on wavelength by making light pass through a material which has a non-constant index of refraction, or by using light in a non-uniform medium such as a waveguide. In this case, the waveform will spread over time, such that a narrow pulse will become an extended pulse, i.e., be dispersed. In these materials, ∂ ∂ ω ω ∂ ∂ k is known as the group velocity and corresponds to the speed at which the peak of the pulse propagates, a value different from the phase velocity.