What is the photoelectric effect and how does it provide evidence for the quantisation of electromagnetic radiation?

(Many students have an intuitive understanding of the photoelectric effect and how it demonstrates quantisation, but it is important when preparing for an exam to be able to explain this clearly and precisely and with the correct key terms)The photoelectric effect is the emission of electrons from the surface of a metal due to incident electromagnetic radiation. It has been observed that the incidence of high intensity visible light on a metal does not induce the photoelectric effect, whereas the incidence of low intensity ultraviolet radiation does. This observation cannot be explained by the wave model of electromagnetic radiation, because this model predicts that energy is continuously supplied by the wave, at a rate given by its intensity, which means that higher intensity radiation should be more successful at expelling electrons (i.e. inducing the photoelectric effect). The fact that this is not what is observed means that an alternative to the wave model is required. The theory that electromagnetic radiation is composed of discrete units of energy (known as photons) provides this alternative model. There are two key features of this model that allow us to explain the photoelectric effect:Each photon has an energy that is proportional to the frequency of radiationPhotons and electrons interact on a one-to-one basis. In other words, an electron will only be emitted from the surface of the metal if it absorbs a single photon that provides it with energy greater than the work function of the metal (which is simply a term for the minimum energy an electron requires to escape from the metal). The electron cannot be emitted simply because it absorbs several photons whose energies add up to the work function.Therefore, this model would predict that, if the radiation incident on the metal is of a frequency low enough that each photon does not have energy greater than the work function, then that radiation cannot induce the photoelectric effect, regardless of its intensity. This is what is seen in our observation - high intensity but low frequency radiation (visible light) does not induce the photoelectric effect while low intensity high frequency radiation (UV) does.(Once the explanation for this basic observation has been understood, we can then go on to talk about other important observations related to the photoelectric effect, such as the KE vs frequency graph, or current vs intensity of radiation)