Classical physics describes light as a wave (electromagnetic radiation) with a set frequency and amplitude where the amplitude is related to the intensity. Light was observed to cause electrons to be ejected from a metal's surface. The classical explanation was that the metal's electrons would oscillate with the light and eventually break away from the surface with a kinetic energy that would depend on the intensity of the incident radiation because the higher the intensity the higher the amplitude of the oscillation. However, the kinetic energy of the ejected electrons was shown to be independent of the intensity of the radiation. In fact, there were some frequencies that no matter how intense the incident radiation was no electrons were ejected.
Einstein modified Planck's concept of quantized energy to describe the experimental results. Einstein proposed that light could travel in small quantized packets of energy (photons) instead of strictly behaving as a classical wave. Einstein showed that the kinetic energy of the ejected electrons was equal to the energy of the incident photon (hν) minus the energy barrier to releasing an electron from that particular metal (workfunction=ϕ). This interpretation is described by the following equation:
KE = 1/2 mv2 = hν−ϕ
This model was able to fully account for the experimental results including the lack of dependence of the energy of the ejected photons on the intensity of the incident radiation as well as the failure of some frequencies of light to eject any photons (the incident energy of the photon was less than the workfunction).
One important result of Einstein's work with the photoelectric effect (outside of the concept of the photon) was the fact that his experimentally determined value of h was the same value determined by Planck. This gave credence to the idea of quantized energy and quantum chemistry as a whole, which was still viewed with suspicion by many scientists.
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