Deterministic simulation, fixed step. Reflection keeps 92% of a spark's energy, a wall 60%, the medium takes 12% a second, a damper keeps 45% and pays 12% of the rest into your budget; below 18 a spark dissipates. Speed is √energy; a prism deviates by 0.6·√(60/energy) radians per face; a field adds a constant acceleration to sparks only. Model, not nature: the numbers are tuned for play and stated so you can reason with them.