Light hits a metal plate. Sometimes electrons fly off, sometimes not — and it isn't about how bright the light is. Adjust the frequency and intensity below and watch what actually decides the outcome.
The line's slope is Planck's constant, h — the same for every metal. Only its starting point (the work function) shifts. That line never bends with intensity, because intensity isn't in the equation.
Why this was strange in 1905: light was understood as a wave, so a brighter wave should always shake loose more energetic electrons. Instead, dim high-frequency light ejected electrons instantly, while intense low-frequency light did nothing at all. Einstein's explanation — that light arrives in discrete packets, each with energy E = hf — helped establish that light behaves as both particle and wave, and won him the 1921 Nobel Prize.