Classical physics predicts that brighter light ejects faster electrons. Experiment says otherwise: below a threshold frequency, no electrons escape regardless of intensity; above it, energy depends on frequency not brightness. Einstein's 1905 explanation — light is quantised — won him the Nobel Prize.
Photons carry energy E = hf (Planck constant × frequency). An electron escapes the metal only if E > ϕ (the work function). Kinetic energy of ejected electrons = hf − ϕ, independent of intensity. Intensity controls the number of photons per second, hence the photocurrent.
Adjust Frequency to cross the threshold and watch electrons begin to escape. Raise Intensity to increase photocurrent without changing electron energy. Switch metal to change ϕ. Observe the live K.E. vs Frequency graph — its slope is Planck's constant h.
When Philipp Lenard measured the photoelectric effect in 1902, he expected wave theory to hold — but it didn't. Einstein's particle explanation was so radical that even after Millikan's precise 1916 confirmation, Millikan himself refused to accept photons were real for another decade.