Rays of different \(\theta\) spread between \(\alpha_2+\Delta\theta\) and \(\alpha_2-\Delta\theta\), equally on both sides of \(\alpha_2\).
From which we conclude color is \(\theta\) and white light has a spread of continuous \(\theta\). Two factors contribute to the spread: change in velocity on crossing into the medium and boundary conditions on \(B\) at the medium boundary.
These factors allows for different colored light to have the same velocity in the second medium and still be refracted to a different refraction angle resulting in the spread of colors. By Snell's Law alone different color lights have different refractive indices and have different velocities in the second medium in order to account for color dispersion.
