Work out photoelectric effect instantly with clear inputs, formula shown and shareable results.
Einstein's photoelectric equation is KE(max) = hf - phi. A 400 nm photon carries 3.10 eV, so on potassium (phi = 2.28 eV) the fastest electrons leave with 0.82 eV and are stopped by 0.82 V. Below the threshold wavelength nothing is emitted no matter how intense the light, which is what killed the classical wave picture.
Photoelectric equation
KE(max) = h c / lambda - phi
Stopping voltage
V(stop) = KE(max) / e
Because each electron absorbs one photon. More photons of insufficient energy still cannot free a single electron - the effect is per-photon, not cumulative.
The potential needed to turn back even the fastest electrons, so it reads the maximum kinetic energy directly in electronvolts.