Gallery of Optics · Exhibit 07
White light is every colour at once. Glass slows violet a little more than red, so each colour turns by a different angle and the beam unfolds into a spectrum.
At each surface the beam obeys Snell's law. The refractive index of glass is not a single number: it rises toward the blue end of the spectrum (Cauchy's approximation), so blue light is bent through a larger angle than red.
Hover the screen on the right to read the wavelength landing there. A second, inverted prism can fold the colours back together.