What is 'dispersion of white light'?

The splitting of white light into its component colours is called dispersion. All the colours travel at the same speed when they travel in a vacuum. But when they hit a medium, such as a glass prism, the colours bend or refract at different angles, and therefore the speed changes. Thus they emerge in different, distinct directions which seem like a spectrum to us. Red colour bends the least and violet deviates the most. In short, we can say that the cause of this splitting of white light into its component colours is due to the different speeds of the colours in the white light when it hits a particular medium. The diagram for dispersion of white light by a glass prism is shown.