Photoelectric Effect - History

The photoelectric effect was discovered by Heinrich Hertz in 1897 while performing experiments that led to the discovery of electromagnetic waves. Since this was just about the time that the electron itself was first identified the phenomenon was not really understood. It soon became clear in the next few years that the particles emitted in the photoelectric effect were indeed electrons. The number of electrons emitted depended on the intensity of the light but the energy of the photoelectrons did not. No matter how weak the light source was made the maximum kinetic energy of these electrons stayed the same. The energy however was found to be directly proportional to the frequency of the light. The other perplexing fact was that the photoelectrons seemed to be emitted instantaneously when the light was turned on. These facts were impossible to explain with the then current wave theory of light. If the light were bright enough it seemed reasonable, given enough time, that an electron in an atom might acquire enough energy to escape regardless of the frequency. The answer was finally provided in 1905 by Albert Einstein who suggested that light, at least sometimes, should be considered to be composed of small bundles of energy or particles called photons. This approach had been used a few years earlier by Max Planck in his successful explanation of black body radiation. In 1907 Einstein was awarded the Nobel Prize in physics for his explanation of the photoelectric effect.