The Photoelectric Effect

The photoelectric effect as a phototube: light of frequency $\nu$ strikes a metal cathode and, if $h\nu$ exceeds the work function $\phi$, photoelectrons are ejected with $K_{\max} = h\nu - \phi$ and drift to the anode under the applied voltage. Below the threshold $\nu_0 = \phi/h$ no electrons appear at any intensity, the result classical wave theory could not explain. Raising the intensity adds electrons but never speeds them up; raising the frequency does. The primary scene is the physical phototube; the side panels are the current-voltage curve (cut off at the stopping voltage, saturating with intensity) and the Einstein line $V_{\mathrm{stop}}(\nu)$ of universal slope $h/e$.

Figure 1. Phototube with photons and photoelectrons, the I-V curve, and the Einstein line.

WHAT TO TRY

Raise the light frequency: below threshold no electrons come off however bright the beam; above it they emerge with energy h nu - phi. That threshold is what classical waves cannot explain.
Crank the intensity: more photons give more current but not more energy per electron, and the stopping voltage stays put. Energy arrives in quanta, not in proportion to brightness.
Dial the retarding voltage to the stopping potential: e V_stop = h nu - phi, the straight line whose slope handed Millikan a value for Planck constant.