Photoelectric Effect Simulator (2D, Stopping Voltage)

This 2D side-view companion isolates the two facts that make the photoelectric effect quantum rather than classical: a hard frequency threshold that no amount of intensity can cross, and a retarding stopping-voltage experiment that measures the true maximum electron energy. Photons approach a cathode plate at a rate set by intensity; below the threshold frequency hf = φ they only produce a dull flash, never an electron, however bright the beam. Above threshold, electrons leave with a real spread of kinetic energies up to hf − φ, fly across a gap, and — if you dial in a retarding voltage — get pushed back before reaching the collector once e·V_stop exceeds their own kinetic energy. Drag V_stop up until the collector current reads zero and compare your measured value against the calculated KE_max/e.