Photoelectric Effect

E = hf = hc/λ — photon energy depends only on frequency, not intensity.

KE_max = hf − φ — excess energy goes to kinetic energy of the ejected electron.

Intensity controls how many photons arrive per second (current), but cannot overcome the work function threshold. This puzzled classical physics — Einstein's explanation won the 1921 Nobel Prize.

Notice: increasing intensity only increases current (more electrons). The maximum kinetic energy is fixed by frequency alone. Below the threshold frequency, no electrons are emitted regardless of intensity.

The stopping potential V_s exactly cancels the electron's kinetic energy: eV_s = KE_max.