Which phenomenon played a crucial role in the early development of quantum mechanics by demonstrating the particle-like behavior of light?

The phenomenon was the photoelectric effect, where shining light on a metal surface ejects electrons only if the light’s frequency exceeds a threshold. This showed that light comes in packets, or quanta, called photons, each carrying energy proportional to its frequency. By observing that the kinetic energy of ejected electrons depended on light frequency and not intensity, scientists realized light behaved like particles. Einstein’s 1905 explanation of the effect convinced physicists that classical wave theory was incomplete, leading to the birth of quantum theory. For example, a 500‑nm light ray can eject electrons from a metal, but a 100‑nm ray cannot, even if both have the same power.