Fermilab

Physicists knew at the time that if one shines ultraviolate rays on various metals, electrons are sometimes emitted. Oddly, no matter how strong the light was, below a certain frequency threshold no electrons were emitted. This threshold depends on the type of metal.

After hearing the idea that light would be quantized into particles called photons, Einstein suggested that an electron in metals receives the energy it needs to leave the metal by absorbing a single photon. Unless the photon's energy exceeds the required amount (called the work function of the metal), the electron cannot leave the metal. Since the photon's energy is given by its frequency, all of a sudden, the existence of the frequency threshold became absolutely natural.

On the next page, I will give a physical description of photons.