The other face of light. Shine light on a metal and it kicks out electrons — but only if the light is blue enough. Red light, however BLINDINGLY bright, kicks out nothing; dim blue light kicks out electrons instantly. That makes no sense for a wave (more energy should help) and perfect sense for particles: one photon, one electron, and only a high-frequency photon carries enough. It won Einstein the Nobel. Slide the colour past the threshold.
Each photon carries energy E=hf. To free an electron costs the work function W, so the ejected electron’s kinetic energy is KE = hf − W — and ONLY if f exceeds the threshold f₀=W/h; below it, no electron leaves however intense the beam (more photons, each still too weak). KE rises with frequency (slope h), never with intensity; intensity sets only the NUMBER of electrons. A wave picture predicts the opposite; the particle picture is exact (Einstein 1905, Nobel 1921). A fail-loud self-check throws unless there is zero emission below f₀ and positive KE above. ◆ real quantum physics, node-verified.
The exact Einstein photoelectric relation (single-photon, one metal); real surfaces add a spread of work functions and multiphoton effects at high intensity — the threshold and the KE=hf−W slope are exact.