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.