Einstein's equation: KE_max = hν − φ; Stopping potential $V_{0}$ = KE_max/e depends on FREQUENCY ONLY — not intensity. Intensity controls photocurrent count, not energy per electron.

Graph slopes: KE_max vs ν slope = h (universal); $V_{0}$ vs ν slope = h/e (universal). Both have x-intercept = ν_{0} (threshold frequency, metal-specific). Parallel lines for different metals.

Photon energy shortcut: E(eV) = 1240/λ(nm). Threshold wavelength: λ_{0}(nm) = 1240/φ(eV). These two shortcuts solve 80% of numerical questions instantly.

Electron de Broglie shortcut: λ = 1.227/√V nm (electrons ONLY). For mass comparison at same V: λ ∝ 1/√m → λ_e/λ_p ≈ 43; λ_e/λ_α ≈ 121.

Davisson-Germer: 54 V, 50°, Ni crystal, 1927 — proved electrons are waves (matter wave-particle duality confirmed experimentally).