Why frequency matters more than intensity
The photoelectric effect reveals that light is quantized. A single photon carries energy proportional to its frequency (E = hf), and if that energy exceeds the work function of the metal, one electron is freed. This threshold behavior is the key insight: shining red light on a metal will never dislodge electrons, no matter how bright, because each red photon carries too little energy. But a single violet photon, despite being dimmer overall, triggers electron emission because its higher frequency gives it more energy. Classical wave theory predicted that brighter light (higher intensity) should knock out more electrons, but it failed to explain why frequency sets a hard cutoff.
Intensity controls electron count, not kinetic energy
Once the threshold frequency is exceeded, increasing light intensity produces more photo-electrons, but their individual kinetic energies stay the same (determined by photon frequency minus work function). This counterintuitive result confirmed Einstein's photon hypothesis and earned him the 1921 Nobel Prize. Modern photodiodes and image sensors exploit this effect: each photon releases at most one electron, and the count of electrons is proportional to photon flux, making them excellent detectors for weak light signals.