Photoelectric effect, quantum light theory

By consideration of Planck s formula W0=Av, Einstein was led to interpret phenomena of another type in terms of the quantum theory, thus giving rise to a new conception of this equation which has proved very fruitful. The phenomenon in question is the photoelectric effect. If light of frequency v falls on a metallic surface,4 electrons are set free and it is found that the intensity of the light influences... 

Albert Einstein s 1905 work on the photoelectric effect paved the way for one of the greatest advances of twentieth-century science, the theory of quantum mechanics. Light had always been regarded as a wave. Quantum mechanics introduced the concept of light being transmitted in wave packets, or photons, that have particle-like qualities as well as wave-like qualities. 

Schrodinger s equation is widely known as a wave equation and the quantum formalism developed on the basis thereof is called wave mechanics. This terminology reflects historical developments in the theory of matter following various conjectures and experimental demonstration that matter and radiation alike, both exhibit wave-like and particle-like behaviour under appropriate conditions. The synthesis of quantum theory and a wave model was first achieved by De Broglie. By analogy with the dual character of light as revealed by the photoelectric effect and the incoherent Compton scattering... 

An apparently quite separate (but in science no two phenomena are really ever unrelated) phenomenon that led to Eq. 4.3, which is to say to quantum theory, is the photoelectric effect the ejection of electrons from a metal surface exposed to light. 

Two papers by Albert Einstein ultimately led to acceptance of the idea of quantization of energy for radiation, and were central to the development of the quantum theory (ironically, in later years Einstein became the most implacable critic of this same theory). The first of these papers, in 1905, concerned the photoelectric effect. Light ejected electrons from a metallic surface if the light had a greater frequency than some threshold frequency v0 which depended on the particular metal. The kinetic energy K of the emitted electrons was proportional to the excess frequency, v — v0 (Figure 5.4). Only the number of emitted electrons, not the kinetic energy, increased as the intensity increased. 

Einstein s explanation of the photoelectric effect was not his only contribution to chemistry. His Ph.D. dissertation, submitted in 1905, was entitled A New Determination of Molecular Dimensions. His investigation of Brownian motion (the random movement of microscopic particles suspended in liquids or gases) was intended to establish the existence of atoms as being indispensable to an explanation of the molecular-kinetic theory of heat. And the concept of relativity has shed light on the motions of electrons in the core orbitals of heavy elements, see also Quantum Chemistry. 

The photoelectric effect was explained by Albert Einstein in 1905 using the principles of quantum physics developed by Max Planck. Einstein claimed that light was quantized— that is, it appeared in bundles of energy. While these bundles traveled in waves, certain reactions (like the photoelectric effect) revealed their particulate nature. This theory was further supported in 1923 by... 

In 1905, only five years after Planck presented his quantum theory, Albert Einstein used the theory to solve another mystery in physics, the photoelectric effect, a phenomenon in which electrons are ejected from the surface of certain metals exposed to light of at least a certain minimum frequency, called the threshold frequency (Fignre 7.5). The number of electrons ejected was proportional to the intensity (or brightness) of the light, but the energies of the ejected electrons were not. Below the threshold frequency no electrons were ejected no matter how intense the light. 

The photoelectric effect conld not be explained by the wave theory of light. Einstein, however, made an extraordinary assumption. He snggested that a beam of light is really a stream of particles. These particles of light are now called photons. Using Planck s quantum theory of radiation as a starting point, Einstein deduced that... 

Using quantum theory, Einstein solved another mystery of physics—the photoelectric effect. Einstein proposed that light can behave like a stream of particles (photons). 

The development of the quantum theory was at first slow. It was not until 1905 that Einstein2 suggested that the quantity of radiant energy hv was sent out in the process of emission of light not in all directions but instead unidirectionally, like a particle. The name light quantum or photon is applied to such a portion of radiant energy. Einstein also discussed the photoelectric effect, the fundamental processes of photochemistry, and the heat capacities of solid bodies in terms of the quantum theory. When light falls on a metal plate, electrons arc emitted from it. The maximum speed of these photoelectrons, however,... 

An apparently quite separate (but in science no two phenomena are really ever unrelated) phenomenon that led to Eq. (4.3), which is to say to quantum theory, is the photoelectric effect the ejection of electrons from a metal surface exposed to light. The first inkling of this phenomenon was due to Hertz, who in 1888 noticed that the potential needed to elicit a spark across two electrodes decreased when ultraviolet light shone on the negative electrode. Beginning in 1902, the photoelectric effect was first studied systematically by Lenard," who showed that the phenomenon observed by Hertz was due to electron emission. 

There is no question that the three reports, the photoelectric effect, the special relativity and the Brownian motion published by Einstein in 1905 changed the traditional consciousness of researchers in the 20th century. The report on the photoelectric effect clarified an old problem in the wave theory of light by linking discussions on the nature of light and the quantum hypothesis originating in Planck s study of specific heat. The report on special relativity refuted the ether theory and created an entirely new field. The report on Brownian motion, aimed at the very existence of atoms and molecules, created the theory... 

Experiments by MilUken in 1908 soon confirmed Einstein s predictions. In 1921, A.H. Compton succeeded in determining the motion of a photon and an electron both before and after a collision between them. He found that both behaved like material bodies in that both kinetic energy and momentum were conserved in the collision. The photoelectric effect and the Compton effect, then, seemed to demand a return to the corpuscular theory of light. The reconciliation of these apparently contradictory experiments has been accomplished only since about 1930 with the development of quantum electrodynamics, a comprehensive theory that Includes both wave and particle properties of photons. Thus, the theory of light propagation is best described by an electromeignetic wave theory while the Interaction of a photon with matter is better described as a corpuscular phenomenon. 

The photoelectron effect was first discovered by Henrich Hertz [11] in early 1887 in order to verify the implications of Maxwell s theory and relations. Hertz noticed a spark of light on metal contacts in electrical units when exposed to light. The dawn of a new era actually came in 1905. Albert Einstein brilliantly utilized Planck s new quantum energy concept to explain how low radiation intensity and high frequency can actually eject electrons from a metal piece. The converse failed to produce any electrons. Max Planck received the Nobel Prize on quantization of energy [12] in 1918 and Einstein received the Nobel Prize on photoelectric effect in 1921. The single relationship proposed so long ago by Einstein is still today the fundamental basis of photoelectron spectroscopy,... 

To explain the photoelectric effect, Einstein assumed that the radiant energy striking the metal surface behaves like a stream of tiny energy packets. Each packet, which is like a particle of energy, is called a photon. Extending Planck s quantum theory, Einstein deduced that each photon must have an energy equal to Planck constant times the frequency of the light ... 

In the quantum theory, energy is quantized, meaning that it can have only certain allowed values. Einstein used the quantum theory to explain the photoelectric effect, the emission of electrons from metal surfaces when exposed to light. He proposed that light behaves as if it consists of quantized energy packets called photons. Each photon carries energy, = hv. 

Einstein s explanation of the photoelectric effect is another step toward the development of the quantum theory. To explain experimental observations, Einstein suggested that light behaves like a bundle of particles called photons. (7.2)... 

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