The Photoelectric Effect Albert Einstein 1879-1955

EARLY in the twentieth century the scientific world had the good fortune to witness the achievements of one of its most remarkable minds. Only when viewed in full historical perspective will the contributions of Albert Einstein assume their total significance in the evolution of scientific thought. 

Nonetheless, for the great reach of his imagination and the power of his conceptual schemes, he had no equal among contemporary scientists. His scientific stature was enormous, both in the eyes of his colleagues and of the public at large. Rarely in the history of physics has a scientist been accorded so much public acclaim. Not since Newton, in fact, was there a physicist who attracted as much universal attention. Indeed, the two are frequently compared on the basis of the great influence each had on the development of physics. In their methods, however, they differed in one major respect: Newton was primarily an experimenter, although his law of gravitation was largely the result of theoretical speculation. Einstein, on the other hand, was a theorist who did no experimental work whatever in the course of his investigations. They shared the distinction of unifying great blocks of knowledge having universal application.

Einstein is perhaps best known for his theories of relativity, yet he worked in many fields, particularly in the kinetic theory of matter and the quantum theory of light. It was in connection with the latter, for his ex-planation of the photoelectric effect,(1) that he won the Nobel prize in 1921. The photoelectric effect was discovered experimentally by Hertz in 1887, in the course of his researches on electric waves. He noticed that the sparks produced in the gap of his secondary or detector circuit were influenced by the light falling upon the gap from the sparks in the primary or transmitting circuit. Upon further investigation, Hertz concluded that it was the ultra-violet portion of the light that was responsible for the phenomenon, and that the effect was greatest when the light was incident upon the negative terminal (cathode) of the gap. Being concerned mainly with other problems, Hertz did not carry these studies very far, leaving to others the more detailed investigations. Many were attracted to the problem, but the most significant contributions were made by Wilhelm Hallwachs (1859-1922), who showed that  the  emission consisted of negative electricity,  and by Lenard, who measured the e/m of the photoelectric carriers and found it to be the same as that determined by Thomson for cathode rays.

By the early part of the twentieth century two empirical laws had been firmly established. First, the photoelectric current, or number of electrons emitted per unit time, was proportional to the intensity of the incident light. Second, the maximum energy of the emitted electrons was proportional to the frequency of the light, not to its intensity. It was at this point, in 1905, that Einstein showed how Planck's new quantum theory of radiation could be used to account for the photoelectric effect. His solution was notable for its simplicity, yet it accounted fully for the observed facts. In that same year he made his first discoveries in the field of relativity.

Albert Einstein was born on March 14, 1879, at Ulm, Württemberg, in South Germany, where his father was a small businessman. Germany was then in a period of rapid economic growth following the Franco-Prussian War and the formation of the empire several years earlier. Bismarck had forged an empire that could not be contained in its own territory; its enormous industrial expansion, with the consequent need for new markets, coupled with notions of racial and cultural superiority, led to two world wars. Einstein was involved in both; in the first, although a Swiss neutral, he supported the pacifist movement. In the second, while he found the thought of war no less deplorable, he could no longer justify his own total pacifism and therefore became instrumental in persuading the United States government to attempt the development of atomic weapons.

His early years were spent in Munich, where his family had settled shortly after he was born. There he attended elementary school and the gymnasium,(2) showing no particular aptitude nor interest in his studies. He enjoyed learning by himself, however, and devoted much of his time to reading in mathematics and science.   When he was fifteen, his family moved to Milan to seek better economic conditions and young Einstein left the gymnasium without taking the examinations required for admission to a university. The following year he attended the Cantonal School at Aarau, Switzerland, where he won his certificate, and then entered the Zurich Poly-technic. There, strangely enough, his chief interest was in experimental physics. He spent a great deal of time in the laboratories, but continued to read ali that he could of the current ideas in physics, studying on his own, for the most part.

Einstein graduated from Zurich in 1900, then did some private tutoring while he acquired Swiss citizenship, and in 1902 was appointed an examiner in the Swiss Patent Office. By his own account his years there were very pleasant; there he found the time to develop his major ideas. During the next two years he published five papers, chiefly on kinetic theory and thermodynamics. These showed marked ability, but in 1905 he achieved true greatness with his papers on the quantum theory of light and on special relativity. That same year he completed a dissertation for his doctorate. By then he was reasonably well known; he began to lecture at Bern Univer­sity, meanwhile retaining his position in the Patent Office. In 1909 he became assistant professor at the Zurich Polytechnic, and two years later accepted an appointment as full professor at the German University in Prague. He returned to Zurich shortly afterward as a full professor, remaining there until 1914, when he was invited to the Kaiser Wilhelm Institute in Berlin, then one of the leading research centers in the world. While there he published his theory of general relativity in 1916, the predictions of which were confirmed by astronomical observations three years later.

As the Nazi mentality began to creep over Germany it became clear that Einstein could no longer remain. German culture was being sacrificed to distorted ambitions, and even scientific truths were suspected of racial taint. Einstein, being a Jew, realized that his own future, both as a scientist and as an individual, was no longer secure in Hitler's Germany and reluctantly decided to leave. In 1933 Einstein joined the Institute for Advanced Studies at Princeton, where he remained for the rest of his life. His presence there enriched the Institute, as well as the entire scientific world. He continued to work quietly, seeking to complete his unified field theory, lending his support to various movements in search of world peace, and condemning ignorance and bigotry wherever he found it. He died on April 18, 1955, shortly before the fiftieth anniversary of his most important discovery, the theory of relativity.

1 Photoelectricity generally includes three distinct phenomena, the photovoltaic, photocon-ductive, and photoemissive effects. The last, with which we are concerned here, is usually called the photoelectric effect.

2 Roughly the equivalem of high school plus junior college.

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Prof. Sérgio Torres
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