Photoelectric Effect

Shine light on a metal and electrons pop out, but only if the frequency is high enough, no matter how bright the beam. Einstein's 1905 explanation (his Nobel prize, not relativity) is bookkeeping with photons: one photon hands its hf to one electron, which spends φ escaping the surface and keeps the rest as kinetic energy. Below the threshold frequency f₀ = φ/h nothing emerges at all.

For cesium, φ ≈ 2.1 eV, so even dim blue light ejects electrons while intense red light does nothing: the observation that classical wave theory could never explain, and the operating principle of every photomultiplier tube.