What underlying principle explains why the photoelectric effect demonstrated the particle nature of light?

The underlying principle is that light energy comes in discrete packets called photons, each carrying energy equal to Planck’s constant times its frequency (E = hν). When a photon strikes a metal surface, its whole energy is transferred to a single electron; if that energy exceeds the metal’s work function, the electron escapes. This shows that light behaves as particles because the ejection depends on the photon’s energy, not on the total light intensity. For example, 400‑nm light (hν ≈ 3. 1 eV) can eject electrons from a metal with a 2.