Evanescent Wave

Physics: Evanescent wave is the exponentially decaying field beyond a TIR interface: E_z ~ exp(−z/δ), where penetration depth δ = λ/(4π√(n₁²sin²θ − n₂²)). As θ → θ_c from above, δ → ∞; at grazing incidence, δ → λ/(4πn₂). TIRF microscopy: evanescent illumination penetrates only ~100 nm above the coverslip, enabling single-molecule sensitivity with negligible background. The Otto and Kretschmann configurations use evanescent waves for surface plasmon resonance (SPR). Quantum tunneling is the exact analog: the particle wavefunction decays exponentially through a classically forbidden barrier region.