Einstein's explanation: light consists of photons with E = hv = hc/lambda. KE_max = hv - phi, where phi = hv_0 = work function (minimum energy to eject electron). Below threshold frequency v_0: no electrons ejected regardless of intensity. Above v_0: KE increases linearly with frequency. Number of electrons ejected per second proportional to intensity (number of photons), NOT frequency. Stopping potential V_0: eV_0 = KE_max = hv - phi. Plot KE_max vs v: straight line, slope = h, x-intercept = v_0, y-intercept = -phi. This proved particle nature of light and gave experimental value of Planck's constant h = 6.626 x 10^-34 J.s.