As visible light penetrates a glass prism from the right, it is refracted, and separated into an array of visible colors.
Each beam of light, with its own particular wavelength (or color), is slowed differently by the glass. Since violet light has a shorter wavelength, it is slowed more than the longer wavelengths of red light. Consequently, violet light is bent the most while red light is bent the least.
This separtion of white light into its individual colors is known as dispersion of light.