CAIE A-Level Physics A2 22.2.3 Photoelectric Emission in Terms of Photon Energy and Work Function Energy Topic Practice

When the surface of a metal plate is illuminated with electromagnetic radiation, electrons are sometimes emitted from the metal.

It is observed that this phenomenon occurs only when the frequency of the electromagnetic radiation is greater than a certain minimum value, regardless of the intensity of the radiation.

Explain how this observation provides evidence for the existence of photons.