How does the photoelectric effect support the notion of wave-particle duality?
When the energy from photons is absorbed by matter, the matter can emit electrons. This process is called the photoelectric effect. The photoelectric effect is a property of light that is not explained by the theory that light is a wave - that is the energies of the emitted electrons during this process are not dependent on the intensity of the incident radiation. The dependence was on frequency instead, ejecting electrons with a specific quanta suggesting to scientist that light should be to treated as both a wave and a stream of particles.
Before 1905 the world of physic has a paradox in their hand. This paradox was the result of treating light as wave. In this classical Maxwell model we would predict that the more intense the incident light the greater the energy with which the electrons should be ejected from the metal. That is, the average energy carried by an ejected (photoelectric) electron should increase with the intensity of the incident light. However experiments showed that this was not so. Rather, it was found that the energies of the emitted electrons to be independent of the intensity of the incident radiation.
Around 1905 Einstein proposed a model that tried to explain why there appear to be no relationship between light intensity and the energies of the emitted electrons during a photoelectric process.
In Einstein's model, increasing the intensity of the incident radiation would cause greater numbers of electrons to be ejected, but each electron would carry the same average energy because each incident photon carried the same energy. This assumes that the dominant process consists of individual photons being absorbed by and resulting in the ejection of a single electron. Einstein's model also stated that it is the increase in the frequency f, that would increase the average energy of the emitted electrons not the intensity, of the incident radiation. This model firmly position light as particle.