dependence of phase velocity on frequency

Dispersion is the idea that the frequency of a wave influences its velocity (speed). Dispersion is most easily seen in light, when all the colors inside white light become separated by a prism.

A diagram of light dispersion through a prism

When a white beam is shone at the prism, all of the colors in the light (colour spectrum) are slowed down when they meet the prism. Each color then refracts (bends) at different angles, making us see a rainbow.

The splitting of white light into seven colours (VIBGYOR) is dispersion

The fastest of the seven colors is red and the slowest violet.