It was around the time that Bohr made his model of the atom, that Albert Einstein hit the scene of physics. Albert was a big supporter of the particle nature of light. Albert not only believed that energy was emitted and absorbed as quantities, but that energy itself was quantized. This simply means that energy itself travels in discreet packets. Einstein called these little particles of light energy photons. Einstein used an experiment done by Philippe Lenard, known as the photoelectric effect, in order to prove that photons exist and that light was a particle.

In Lenard's experiment there is a metal target that is bombarded by light. When the light hits the plate electrons are knocked out of their shells in the atoms. With sensitive instruments the number of electrons and their velocities are measured. Lenard found that the electrons were being emitted immediately after the light hits the plate. Lenard also found that dimming the light source effected the number of electrons emitted in a certain time. The dimmer the light the less electrons. The experiment showed that the dimming had no effect on the velocity of the electrons. The only thing that was found to effect the velocities of the electrons was the color of the light.

Einstein used all of these as proof for his theory of light. In Lenard's experiment the electrons were emitted immediately after the light struck the plate. Einstein said this was proof that light was a particle because only a particle could knock off an electron immediately upon impact. It was like two billiard balls colliding. The photon hits the electron and sends it flying off immediately in another direction. If light were a wave the emission of electrons would have to be delayed because it would take a few oscillations before the electron had enough energy to leave orbit. It would be like a piece of drift wood in the sea. It takes a couple of waves before the wood is pushed ashore. Dimming the light is also proof that light is a particle, because if less photons are hitting the plate it only makes sense that less electrons will be emitted. Finally, the fact that color effects the velocity of the rebounding electrons also proves that light itself is quantized. Each photon carries a specific amount of energy that corresponds to a color. A red photon will have less energy than a blue photon. This was a huge leap forward in the development of quantum mechanics. It is for this idea and not his theory of relativity that Einstein received the Nobel prize in 1923. The pendulum seemed to swing and now light was definitely a particle. All this was quite good except there was one catch, the dreaded Two Slit Experiment.