The Photoelectric Effect

Armando Gonzalez

The photoelectric effect, originally observed in 1839, but documented by Heinrich Hertz in 1887, consists on the emitting of electrons when a light source hits a surface. These emitted electrons are called photoelectrons. Phillip Eduard Anton von Lenard observed that that the energy of these electrons was proportional to the frequency. This theory contrasted James Clerk Maxwell’s wave theory of light, a theory that prevailed during this period of 1800’s. Von Lenard’s theory proposed that the energy of the electron was proportional to the intensity and in that case inversely proportional to the frequency. Albert Einstein solved this dilemma by proposing that light is composed of photons rather than waves and that the energy in these photons was equal to the frequency times a constant later called Planck’s constant. This theory granted him his first Nobel Prize in Physics in 1921.

For a photon to eject an electron it needs to reach a certain frequency where it will have enough energy to release an electron. To show this effect, a vacuum chamber needs to be created with the conductive metal at one end and a collector at the other. When a light shines on the metal, the electrons are released and move toward the collector. This creates a current that can be measured. The point at which no electrons make it to the collector is called the stopping potential and can be used to determine the maximum kinetic energy of the electrons. Not all of the electrons will have the same energy; they will be emitted with different energies depending on what metal is being used. Also the type of light in use is a factor since some lights have more frequency than others. The kinetic energy, in other words, is the energy of the particles knocked free of the metal surface with the greatest speed. Some of the uses for this effect include night vision devices, spacecrafts, image sensors, photoelectron spectroscopy and others.

The experimental results of the photoelectric effect are:

-the rate of the ejected electrons is directly proportional to the intensity of the light source

-there exists a minimum frequency for any type of metal at which photoelectrons can be emitted.

-an increase in the frequency of the light source causes the photoelectrons intensity to increase too

-the kinetic energy of a emitted photoelectron depends on the frequency of the light source but not on the intensity as long as it is not very high

-the time for the emission of a photoelectron is very small

-the direction of the electrons depends on the direction of the polarization of the light source

Although the relation between the energy of the electrons and the intensity was not tested until 1915, almost 10 years after Einstein proposed this relation. I think it is amazing how Albert Einstein solved the problem of the photoelectric effect with such a simple statement. By proposing photons or light quanta, he proved wrong a theory that had been used for many years.