Albert Einstein postulated that the process by which electrons are liberated from a metal surface by incident light can be understood if the light is considered to be composed of particles called light quanta, which later became known as photons.
The photoelectric effect is the process by which electrons are ejected from a metal surface when light of the appropriate frequency is shined on that surface. Because it requires energy to remove an electron from a metal, it is clear that this energy is coming from the incident light. In 1905, a number of mysteries were associated with this process, all of which were solved by Einstein’s explanation of the photoelectric effect in terms of light quanta.
By 1905, scientists had discovered a variety of physical phenomena associated with the photoelectric effect. In 1887, Heinrich Hertz had discovered that light incident on a metal surface can produce visible sparks if that surface is in the presence of an electric field. The sparking action demonstrated that something was being removed from the metal surface, although nobody knew what it was. In 1888, Wilhelm Hallwachs
The photoelectric effect posed serious problems for classical physics. According to the classical theory, light was an electromagnetic wave that carried energy based on its intensity. When this energy was transmitted to the irradiated body, the electrons in the body would gain energy gradually, or “heat up,” until eventually they became energetic enough to escape from the body. If the incident light was very intense, the electrons should be escaping with a large supply of energy. The experimental observations were inconsistent with this explanation, however; they showed that the energy of the ejected electrons depended on the frequency of the incident light but not on its intensity. Yet the question of why a dim source of high-frequency light resulted in the ejection of electrons with a higher energy than a bright source of low-frequency light still remained.
In 1905, Albert Einstein published three revolutionary papers. The most famous was on relativity, another was on Brownian motion
As an aid to understanding, Einstein postulated that the incident light of the photoelectric effect should be viewed not as a classical wave but rather as a collection of particles, which he called light quanta and which were later renamed photons. Each of these photons carried a discrete amount of energy that was proportional to its frequency: E = hf, where E is the energy of the photon, f is the frequency, and h is a proportionality constant that had been discovered by Planck. In Einstein’s conception, a beam of light is more closely analogous to a flock of birds than to a stream of water.
By viewing the incident light as a collection of photons, Einstein was able to explain the photoelectric effect as follows: When a photon is incident on a metal surface, there is a strong chance that it will penetrate through the surface and encounter the free electrons that are known to lie within the metal. When a photon encounters an electron, it will typically transfer all of its energy to the electron. In the language of modern physics, it is said that the photon is “absorbed” by the electron. In general, an electron can absorb only one photon, but it will always absorb this photon in its entirety. The electron, which had very little energy before it absorbed the photon, now has an amount of energy hf. If this energy is high enough, the electron will be able to escape out of the metal. An electron will typically expend a certain amount of energy as it escapes. This energy is characteristic of the specific metal and is known as its work function, P. The work function is the “energy cost” of the escape. If the electron is going to escape, then the energy provided by the photon, hf, must be greater than P. Einstein proposed the formula E = hf – P, which states that the energy possessed by a photoejected electron is equal to the energy of the incident photon minus the energy of the work function. By analogy, one could say that the money possessed by an escaping convict is equal to the money that was smuggled in to arrange his escape less the amount he spent bribing relevant prison officials to arrange his escape.
Einstein postulated the curious relationship between the energy of the ejected electrons and the incident light: First, the intensity is irrelevant if an electron absorbs only a single photon. Higher intensity means more photons that might eject more electrons but will not increase the energy of any specific electron because, at most, one photon is absorbed. Second, the energy of the ejected electrons increases with the frequency of the incident light, because higher-frequency photons impart more energy to the electrons. The photoelectric effect was no longer a mystery.
Einstein was also able to make two predictions: The energy of a photoejected electron can never exceed hf, the energy of the photon, and, if hf is less than the work function of the metal P, no electrons will be ejected no matter how intense the incident light. Einstein’s predictions were experimentally verified.
Einstein’s explanation for the photoelectric effect came at a time when classical ideas were still strong and the notion of light quanta seemed radical and mysterious. Even Planck had never fully accepted the reality of the quantum that he had discovered in 1900. Einstein, by calling his ideas “heuristic,” indicated that he had reservations about the physical reality of the light quanta. In fact, it was almost two decades before these important ideas were universally accepted.
The first three decades of the twentieth century witnessed the overthrow of classical physics and the birth of quantum mechanics, which has been the foundation for most of the physics developed since that time. Like many revolutions in science, the quantum revolution was accomplished through a series of small steps that eventually led to an entirely new way of looking at the universe. Einstein’s explanation of the photoelectric effect in 1905 was one of the small steps along the road to the radically new world into which physics was about to enter. The light quanta hypothesis became an important part of several larger theories. In 1911, Niels Bohr
In 1923, Arthur Holly Compton
The idea of light quanta was slow to catch on, however. In 1911, Einstein still was calling attention to the tentative nature of his hypothesis. Even as the explanation was verified experimentally and became widely accepted, there was still hesitation about the physical reality of light quanta. As late as 1924, Bohr coauthored a paper that still argued that the photon was not real and that the concept would eventually be replaced by an improved understanding of matter and radiation. Nevertheless, it was rapidly becoming clear to the physics community that the quantum revolution had arrived. Eventually, all the opposition ceased, and the photon became one of the most important concepts in physics, universally accepted, and a model for later developments in other areas of physics.
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Gamow, George. Thirty Years That Shook Physics: The Story of Quantum Theory. Garden City, N.Y.: Doubleday, 1966. This classic book by one of the principal physicists of the twentieth century is a charming and insightful survey of the revolution that produced the modern theory of quantum physics. -
Halliday, David, and Robert Resnick. Fundamentals of Physics: Extended Version. New York: John Wiley & Sons, 1988. This popular general physics text is typical of the many excellent books that discuss the photoelectric effect in their treatment of elementary modern physics. -
Pais, Abraham. Subtle Is the Lord: The Science and the Life of Albert Einstein. Reprint. New York: Oxford University Press, 2005. This highly acclaimed book, first published in 1982, is written by a physicist who knew Einstein very well. Somewhat technical in parts, it nevertheless presents an absolutely authoritative discussion of Einstein and his ideas. -
Rigden, John S. Einstein 1905: The Standard of Greatness. Cambridge, Mass.: Harvard University Press, 2005. An account of the new insights and turmoil engendered among physicists by the five groundbreaking research papers that Albert Einstein published in 1905. Accessible to lay readers. Includes simple diagrams and reproductions of the front pages of the five papers. -
Rosenthal-Schneider, Ilse. Reality and Scientific Truth: Discussions with Einstein, von Laue, and Planck. Detroit: Wayne State University Press, 1980. Consists of correspondence and discussions with three eminent scientists: Albert Einstein, Max von Laue, and Max Planck. Topics covered include “the universal constants of nature,” “concepts of substance and conservation,” and “the smallest length.” -
Wolfson, Richard, and Jay M. Pasachoff. Physics: Extended with Modern Physics. Glenview, Ill.: Scott, Foresman, 1989. Chapter 36, “Light and Matter: Waves or Particles,” does a very complete job of explaining the photoelectric effect and its significance for physics.
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