What is the photoelectric effect? (A-Level Physics)

The photoelectric effect states that electrons are emitted when electromagnetic radiation is shone upon a material. It also provides evidence for the particle-nature of light, since each photon interacts with only one electron.We know that each photon interacts with only one electron because changing the intensity (how much energy each photon has) of the radiation doesn't change the number of electrons that are emitted. The only way to increase the number of electrons that are emitted is to increase the number of photons that are shone on the material.