Laser Cavity Threshold Lab (2D)

This 2D companion runs the same physics as the 3D optical-cavity model — a pumped gain medium between two mirrors — but through a plain canvas view built to make the lasing threshold itself the point: real four-level rate equations for population inversion and photon number are integrated live, mirror reflectivities and internal loss set the cavity's round-trip loss γ, and a running graph shows the inversion clamp and the photon-number kink the instant pump rate crosses the computed threshold P_th, instead of leaving the threshold as an implied backstory to a 3D beam.