2.5.3 The Third Failure: The Photoelectric Effect
In 1887 Heinrich Hertz discovered that ultraviolet light shining upon a metallic plate induces emission of particles from it later identified by Philipp von Lenard as electrons. This experience was quickly studied experimentally by Lenard himself who found that the energy of the electrons released from the surface of the metal was independent of the intensity of the light which produced them. What did depend however upon the intensity of light was the number of electrons extracted by it. While one could easily explain the emission of electrons as an extraction of them from the atoms of the metal by the energy of the impinging light, one could not reconcile the experimental results with the basic conceptions underlying the theory which governed light and its interaction with electric charges. Thus, if light extracted the electron by virtue of the energy it transferred to it, why is it that the energy of the resulting electron does not depend upon the intensity of extracting light? After all, it stands to reason that after expending some amount of energy in the process of extracting the electron from the metal, the higher the energy left in the light wave the more energy should be transferred by that wave to it. Even more puzzling, Millikan, following a 1905 suggestion by Albert Einstein, experimentally found in 1916 that the energy of the photoelectron which is in excess of that required to extract it from the metal is directly proportional to the frequency of the light employed. By what mechanism does the frequency of the impinging light affect the energy of the resulting electron? And why is it that the number of electrons, rather than their energy, increases with the energy of light? Since no cogent answers could be found within the then current corpuscular theory, the photoelectric effect stood as one more uncomfortable reminder of the storm of failures gathering around the scientific community at the turn of the 19th Century.