The Photoelectric Effect 

The details of the photoelectric effect were in direct contradiction to the expectations of very well developed classical physics.The explanation marked one of the major steps toward quantum theory.The remarkable aspects of the photoelectric effect when it was first observed were:

                1. The electrons were emitted immediately - no time lag!

                2. Increasing the intensity of the light increased the number of photoelectrons, but not their maximum kinetic energy!

                3. Red light will not cause the ejection of electrons, no matter what the intensity!

                4. A weak violet light will eject only a few electrons, but their maximum kinetic energies are greater than those for intense light of longer wavelengths!

What did that mean?

Analysis of data from the photoelectric experiment showed that the energy of the ejected electrons was proportional to the frequency of the illuminating light. This showed that whatever was knocking the electrons out had an energy proportional to light frequency. The remarkable fact that the ejection energy was independent of the total energy of illumination showed that the interaction must be like that of a particle which gave all of its energy to the electron! This fit in well with Planck's hypothesis that light in the blackbody radiation experiment could exist only in discrete bundles with energy.

Article Provided by Hyperphysics 

Davisson Germer Experiment 

This experiment demonstrated the wave nature of the electron, confirming the earlier hypothesis of deBroglie. Putting wave-particle duality on a firm experimental footing, it represented a major step forward in the development of quantum mechanics. The Bragg law for diffraction had been applied to x-ray diffraction, but this was the first application to particle waves.

Article provided by Hyperphysics

How this experiment did this is by sending a stream of electron into a crystal. Because crystals have very well structured atomic lattices (atoms are spaced out uniformly in the solid).  The beam of electrons would be deflected if the deBroglie waves is larger then the spacing between the atoms 

Franck Hertz Experiment 

      Electrons are accelerated in the Franck-Hertz apparatus and the collected current rises with accelerated voltage. As the Franck-Hertz data shows, when the accelerating voltage reaches 4.9 volts, the current sharply drops, indicating the sharp onset of a new phenomenon which takes enough energy away from the electrons that they cannot reach the collector. This drop is attributed to inelastic collisions between the accelerated electrons and atomic electrons in the mercury atoms. The sudden onset suggests that the mercury electrons cannot accept energy until it reaches the threshold for elevating them to an excited state. This 4.9 volt excited state corresponds to a strong line in the ultraviolet emission spectrum of mercury at 254 nm (a 4.9eV photon). Drops in the collected current occur at multiples of 4.9 volts since an accelerated electron which has 4.9 eV of energy removed in a collision can be re-accelerated to produce other such collisions at multiples of 4.9 volts. This experiment was strong confirmation of the idea of quantized atomic energy levels.