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Einstein's equation: KE_max = hv - phi = h(v - v_0). phi = hv_0 = work function (minimum energy for ejection). Below v_0: no electrons ejected regardless of intensity. Above v_0: KE increases linearly with v. Number of electrons proportional to intensity (not frequency). Stopping potential: eV_0 = KE_max. Graph of KE vs v: straight line, slope = h, x-intercept = v_0, y-intercept = -phi. Energy shortcut: E(eV) = 1240/lambda(nm). Threshold wavelength: lambda_0 = 1240/phi(eV). Key distinctions: frequency determines KE (energy per photon), intensity determines current (number of photons per second). Photoelectric effect proved the particle nature of light. It cannot be explained by wave theory because: (1) there's a threshold frequency, (2) KE depends on frequency not intensity, (3) emission is instantaneous. Compton scattering provided further evidence of photon momentum.