"White light, is actually made out of light of several frequencies (colors) travelling together. These basic frequencies of visible light are part of what we call the visible spectrum, and it is only tiny part of the entire electromagnetic spectrum.
As white light enters a medium (in this case, the prism), each of its composing wavelengths will travel at a different speed in the new medium, This is the phenomenon we call refraction.
Since light of different wavelengths will change direction by a different amount, we will experience a division of white light in its composing spectral colors, represented here by colored waves. This is what we call dispersion.”
Images: (1, 2, 3)