Figure 1: The spreading of light
The change in the propagation constant for different waves is called dispersion. Dispersion occurs because the spreading of a wave is dependent on the wavelengths' interaction with the refractive index of the medium. Different wavelengths travel at different speeds in the medium. Material dispersion is a function of the source spectral width.
Basically, dispersion is the spreading of light as light propagates through a more dense medium. The speed difference of waves of different frequencies shows oneself particularly clearly in some media e.g. at a prism from glass: white sunlight is taken to pieces into his spectral colors in this.
This phenomenon leads to the rise of the rainbow by water droplets in the atmosphere.