 The photoelectric effect is the ability of light to dislodge electrons from a metal surface. The effect was discovered in 1887, and the emitted electrons are called photoelectrons. In 1915, Robert Milliken developed an experiment to study this effect. Here's a virtual reproduction of his photoelectric effect experiment. A vacuum tube contains two plates connected to an external circuit that produces a voltage between the plates to oppose the flow of electrons. When a light source shines on the emitting plate, energy is transferred from the light to electrons in the plate. If an electron gains enough energy to overcome the plate's binding energy, it will be dislodged. Furthermore, if such an electron has enough additional kinetic energy left to overcome the voltage, it will reach the other plate. This is then measured as an electric current. As the voltage is increased, the number of electrons that can make it across goes down. At some point, the voltage is large enough so that only the most energetic electrons can make it across. Any additional increase will stop all electrons and the current will stop. This is called the stopping potential. And the energy of those most energetic electrons is the maximum kinetic energy. Classical wave theory predicted that light energy would take some time to build up in the electrons before they can escape. And the maximum kinetic energy of the electrons would be proportional to the intensity of the light that shines on the metal, no matter what the light frequency might be. But what we actually see is that although the number of electrons varies with the light intensity, the maximum kinetic energy of these electrons remains the same. In addition, electrons are emitted without any delay. Except that for really low frequency light, no electrons are emitted at all, no matter how intense the light. To find out what is actually going on, Milliken measured and graphed the max electron energy for varying light frequencies. Here are his six definitive data points. They create a straight line. The ratio of the electron energy to the light frequency is a constant. Careful measurement found that this constant was equal to Planck's constant developed earlier from black body radiation. Ten years earlier, in 1905, Einstein had proposed that the light impacting the plate was quantized into chunks he called quanta, or photons. A photoelectron is released as a result of an encounter with a single photon. The entire energy of the photon is delivered instantaneously to a single photoelectron. If the photon energy is large enough, the photoelectron will be released. This explained all the photoelectric effect observations. Ten years later, Milliken, much to his own surprise, proved that Einstein was correct.