Send light the other way — from dense glass UP into air. It bends away from the normal, but can it always escape? Is there an angle past which none gets out?
▶ Launch the interactive simulationSweep the incidence angle θ₁ for light going glass (n₁=1.5) → air (n₂=1.0). At each angle draw the incident, reflected and refracted rays, dimming each by its Fresnel share of the energy, and measure the critical angle by bisection (the θ₁ at which the transmitted ray reaches grazing, sinθ₂=1).
SNELL + the CRITICAL ANGLE — θ₂ = asin((n₁/n₂)·sinθ₁) climbs to 90° at θ_c = asin(n₂/n₁) = 41.8°, beyond which sinθ₂ > 1 is impossible: NO transmitted ray exists and the reflection is total (the bisection-measured θ_c matches asin(n₂/n₁) to ~1e-11)