• Tags: photoelectric, observations, experiment
• Difficulty: Foundation

Hertz (1887) discovered the photoelectric effect; Einstein (1905) explained it using the photon concept. Key experimental observations: (1) No emission below the threshold frequency $f_{0}$, regardless of how intense the light is. (2) Above $f_{0}$, emission is instantaneous (< 10^{-9} s delay). (3) Maximum KE of emitted electrons depends only on frequency, not intensity. (4) Photocurrent (number of electrons per second) is directly proportional to light intensity. (5) Stopping potential $V_{0}$ is independent of intensity but increases linearly with frequency.

Classical wave theory could not explain observations (1), (2), and (3). It predicted that increasing intensity should eventually provide enough energy for emission at any frequency, and that there should be a time delay for energy accumulation. Einstein's photon theory explained all observations.