Stopping Potential

Stopping potential V_s is the retarding voltage (anode held NEGATIVE w.r.t. cathode) at which photoelectric current drops to ZERO.

At V = V_s, even the MOST energetic photoelectron (K = K_max) is just barely stopped: eV_s = K_max.

Stopping-potential equation: eV_s = hf − φ.

V_s is INDEPENDENT of intensity — depends only on frequency and metal.

Plotting V_s vs f: straight line with slope h/e and x-intercept f₀.

Slope h/e ≈ 4.136 × 10⁻¹⁵ V·s — same for all metals. Y-intercept (at f = 0): −φ/e — depends on metal.

Millikan's 1914 experiment measured V_s vs f for several metals — confirmed Einstein's equation and gave a precise value of h.