Fresnel constructive thickness: d(θ=0)=λ/(2n_f); d(θ=π/3)=λ/n_f

Sympy-exact witness of the thin-film constructive-interference thickness formula at m=1. Setup: d = λ/(2·n_f·cos(θ)). Identity 1 (normal incidence, θ=0): cos(0)=1, so d = λ/(2·n_f) — the canonical quarter-wave (as λ_film = λ/n_f, d =…