Each photon carries a fixed energy E = hf, set entirely by its color (frequency) — not its brightness. An electron is only knocked loose if a single photon's energy exceeds the metal's work function. Turning up the intensity slider fires more photons per second (more current, if any electrons are emitted at all), but it can never make a single too-weak photon eject an electron — a fact classical wave theory could not explain, and one of the reasons Einstein proposed that light itself is quantized.