How do we know the energy of a photon (light particle) is quantised?

Firstly, quantisation of energy means that energy can only exist for specific, discrete values, i.e. it is not continuous. Now, photons display this phenomena however it is not immediately obvious why, as in classical physics, we observe that light acts as a wave. To understand how the we know these particles are quantised we have to investigate a some practical examples. One of these examples is called the photoelectric effect. This is where a electromagnetic radiation (light) is incident on a metal. This causes electrons to become excited and thus jump out of the metal under certain conditions. However as the intensity of the light increases the energy of the electrons does not, leading us to believe that the light is not behaving as a classical wave as first thought. Furthermore, we observe that no electrons are emitted if the frequency of the light is below a certain value, called the work function. These two observations lead us to draw the conclusion that light, in this experiment, is acting a particles of discrete energy.