Total Internal Reflection
critical angle · evanescent wave · frustrated TIR
Physics: Critical angle θ_c = arcsin(n₂/n₁); for glass/air (n₁=1.5): θ_c = 41.8°. Beyond θ_c, all light reflects — total internal reflection. Evanescent wave penetrates medium 2 with E ~ exp(−z/δ), where δ = λ/(4π√(n₁²sin²θ − n₂²)). Frustrated TIR (FTIR): bring a second medium within δ and light tunnels through — exact optical analog of quantum tunneling. Goos-Hänchen shift displaces the reflected beam laterally. TIRF microscopy uses evanescent illumination (~100 nm depth) for single-molecule imaging.