What is the photo-electric effect and what impact did it have on the development of Quantum Mechanics?

The photo-electric effect is the process in which a metal will emit electrons when light of a sufficiently high frequency is incident upon it, Before Einstein's description of the photo-electric effect the observed emission of electrons from such metals was a paradox in physics because the wave theory of light used by Maxwell's equations (the working model for light at the time) implied that the energy of emitted electrons should increase with increasing intensity of light, however, experiment showed that the energy of electrons emitted was independent of the intensity of the incident light. Einstein's solution to this paraox was to model light as a particle, called the photon, and then to show that the energy of photons is linearly proportional to their frequency (E=hf). Then photons with high enough energy would cause the electrons in the metal to "excite" and leave the metal Then, while increasing intensity of light would increase the number of emitted electrons, because more intense light has more photons, the energy of the electrons is related to the frequency of the light, not the intensity. Einstein's description also explained why the photo-electric effect was not observed for low frequency light; the photons did not have enough energy to excite the electrons in the metal. Experiment agreed with Einstein's model: a historically significant result because it became one of the main arguments for the wave-particle duality of light.