Photoelectric Effect Simulator (2D, Stopping Voltage)

Light source

Frequency 6.5×10¹⁴ Hz

Intensity (photon rate only) 5

Cathode material

Stopping-voltage experiment

Retarding voltage V_stop 0.00 V

Raise the frequency past threshold to eject electrons.

Live readout

Photon energy hf

— eV

Work function φ

— eV

KE_max = hf − φ

— eV

Threshold

—

Collector current

0.00 µA

Electrons/s emitted

How this experiment works

Every photon carries a fixed quantum of energy E = hf. An electron at the cathode surface absorbs exactly one photon; if hf exceeds the metal's work function φ, it escapes with kinetic energy anywhere from 0 up to KE_max = hf − φ (real photoelectrons leave with a spread of energies, not all at the maximum). Below the threshold frequency hf = φ, raising the intensity only throws more sub-threshold photons at the plate — never any electrons, because no single photon carries enough energy alone.

The retarding field between the plates removes energy from every electron crossing the gap, uniformly per unit distance. When V_stop is raised, slower electrons turn back first; only when e·V_stop reaches KE_max do even the very fastest electrons fail to cross, and the collector current drops to zero. That voltage is Millikan's stopping potential, and it equals KE_max/e regardless of intensity — which is exactly why intensity changes the current but never the stopping voltage.

Cathode (left) → retarding gap → collector (right)