22.2.5—Why the maximum kinetic energy of photoelectrons is independent of

For photoelectrons, the maximum kinetic energy is set by the energy of one incident photon: K_max=hf−Φ.

At a fixed frequency above threshold, raising intensity sends more photons per second, so the photocurrent can rise, but each photon still supplies the same energy and K_max is unchanged.

Doubling the intensity of ultraviolet light doubles the possible emission rate in an ideal experiment, not the stopping potential.

Intensity is not a substitute for frequency: sub-threshold photons do not eject electrons simply because the beam is brighter.