PHOTOELECTRICITY. The photoelectric effect is the name given to the power shown by light, X-rays and y-rays of causing a body to emit electrons, or negative electricity. Light, X-rays and y-rays are all electromagnetic radiations differing only in their wave-lengths, that of light being the longest ; and it is the electric forces in the radiation, acting on the electrons in the atoms of the body, which communicate sufficient energy to the electrons to set them free from the atom, and even to give them very high velocities. The phenomenon is interesting and important from several points of view, of which the chief is that it repre sents one of the three fundamental methods of interaction be tween radiation and matter, the other two being resonance-absorp tion and scattering. The laws governing this phenomenon are simple to express although difficult to understand, and maintain their validity over an enormous range of wave-length. Ordinary light has a wave-length one million times as great as that of the short 7-rays, but yet the relation governing the speed of the ejected electron is the same in both cases.
The study of the photoelectric effect has raised many points of great interest in connection with our views as to the real nature of radiation, since it brings to light a behaviour apparently irrec oncilable with the simple continuous wave-theory used to explain interference. While this disagreement still defies explanation, the effect is sufficiently understood to be of the greatest use in elucidat ing the structure of the atom. The reason for this lies in the fact that the action of the radiation on an atom is selective. While long wave-length light leads to the ejection of electrons from the outer portions of the atom, the much shorter wave-length X-rays and 7-rays interact chiefly with the innermost electrons. From the nature of this interaction it is possible to obtain information successively about the different electrons in the atom.
The photoelectric effect is not only important from the purely scientific standpoint, but also finds a most important practical application in the construction of the photoelectric cell, which is one of the most trustworthy methods existing for measuring the amount of energy in a beam of light. It has rendered possible the construction of accurate photometers for determining the density of photographic plates, and has given methods for measuring the amount of light emitted by extremely faint stars.
The action of the radiation being to cause the ejection of elec trons from atoms, it will be seen that the primary questions re garding each individual process are how fast the electron is ejected and in what direction, and to this must then be added the question of how many electrons are set free each second when radiation of given intensity is incident on matter. More or less satisfactory answers can now be given to these questions, and the history of the subject is best described by reference to those researches which showed how the number and velocity of the electrons depended on the intensity and wave length of the radiation.
In the following years a great deal of work was carried out on this peculiar action of light, and attention was first directed to the intensity of the emission and how it depended on the state of the surface of the metal and on the polarization of the light. In 1902 Lenard opened up a new method of investigation by directing attention to the velocity of the ejected electrons. He was able to show that the maximum velocity of the electrons was inde pendent of the intensity of the light but did depend on the wave length. He appreciated quite clearly the difficulties this raised in the way of any explanation, since the obvious suggestion of the ejection being due to a transference of energy from the light fo the atoms of the metal by some resonance action was rendered impossible by its lack of dependence on the intensity. It was this work which formed the basis of Einstein's famous paper of 1905. Einstein suggested that the general phenomenon of the conversion of light could be better understood on the assumption that the en ergy of light radiation was not distributed continuously in space. As radiation spread out from a source he imagined it to remain localized in small bundles, or energy quanta, which became fur ther and further apart as they receded from the source, whilst individually remaining unchanged. He pointed out that when an atom absorbed radiation the simplest assumption would be that a light quantum gives its whole energy to an electron. It was nat ural to associate the energy of the light bundles with the definite amounts of energy postulated by Planck in his quantum theory of black body radiation. Hence Einstein suggested that, at every act of absorption, the energy received by the electron was he', v being the frequency of the radiation and h a universal constant which should be identical with that occurring in Planck's formula for the black body radiation.