Light Through a Medium 2D: Phase vs Group Velocity & Dispersion

This is the 2D companion to the 3D Quantum Light Propagation Through a Medium scene. Instead of decorative spinning primitives, it computes a real wave packet crossing from vacuum into a tunable refractive medium: the carrier oscillates at the phase velocity c/n(λ) while the Gaussian envelope advances at the group velocity c/N(λ), with the group index derived analytically from a Cauchy dispersion relation. Switching to the white-pulse view launches red, green and blue components together and lets each refract and travel at its own speed, visibly spreading the pulse into its component colors exactly as a prism splits white light — a genuinely different rendering technique and a genuinely different physics model from the 3D scene's placeholder geometry.