Einstein's explanation: light consists of photons with E = hv = $\frac{hc}{lambda}$. $KE_{max}$ = 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.