Lenard, Philipp von (physicist)


(1862-1947) Hungarian/German Experimental and Theoretical Physicist, Atomic Physicist

Philipp von Lenard is most famous for developing an ingenious experiment involving a cathode ray tube with a thin aluminum window that permitted the rays to be studied as they escaped into the open air. The results of this research, which led him to conclude that the volume of an atom is mainly composed of empty space, contributed to the conception of Rutherford’s planetary model of the atom and earned Lenard the 1905 Nobel Prize in physics. His pioneering work on the photoelectric effect was influential in albert einstein’s formulation of the theory of light quanta.

He was born in Pressburg, Hungary (now Bratislava, Slovakia), on June 7, 1862. His family was originally from the Tyrol, and Lenard was strongly drawn to German culture. After briefly studying at the University of Budapest, he went to Germany, where he studied physics at the University of Heidelberg under Robert Bunsen, the inventor of the Bunsen burner, and at the University of Berlin under the eminent hermann ludwig ferdinand von helmholtz. In 1886, he received a Ph.D. from Heidelberg, where he remained for the next three years.

In 1891, Lenard became assistant to hein-rich rudolf hertz, the discoverer of radio waves, at the University of Bonn, where he did his first original research in mechanics, on the oscillation of precipitated water drops. He also worked on luminescence and phosphorescence. He then began his work on cathode rays, the phenomenon that produced a fluorescent glow when most of the air was pumped out of a glass tube with wires embedded at either end and a high voltage was sent across it. At the time physicists were asking whether cathode rays were charged particles or some undefined wavelike process in the ether. In 1892, after reading William Crookes’s 1879 paper on the movement of cathode rays inside discharge tubes, Lenard became interested in designing an experiment that would allow cathode rays to be examined outside discharge tubes. Hertz suggested that Lenard construct a cathode ray tube with a thin sheet of aluminum serving as a window, now called the Lenard window, to contain the vacuum, while releasing the cathode rays. Lenard experimented with windows made of aluminum foil of varying thickness and in 1894 published results that revealed the amazing discovery that the cathode rays could move about 8 cm in the air after passing through the thin aluminum window. In this experiment he found that the cathode rays decreased in number as the distance from the tube increased and was able to show that the ability of a material to absorb cathode rays depends on its density. The experimental finding that cathode rays were able to pass through the aluminum foil led him to the hypothesis that the volume that the atoms occupied in the metal consisted of a large amount of empty space. This result proved crucial to joseph john (j. j.) thomson in his discovery that cathode rays were really electrons: particles that had to have a mass much smaller than the mass of any atom. Lenard had hoped to make that discovery himself and was embittered when Thomson beat him to it. Subsequent experiments by others measured the charge directly and confirmed Lenard’s conclusions.

After his groundbreaking work on cathode rays, Lenard accepted an associate professorship at the University of Breslau and in 1895 became professor of physics in Aachen. In 1896, he became professor of theoretical physics at Heidelberg, and in 1898, he became professor of experimental physics at the University of Kiel.

In 1901, on the basis of his cathode ray experiments, he concluded that the part of the atom where the mass was mostly concentrated consisted of neutral doublets, or “dynamides,” of negative and positive electricity, which were very small and separated by wide spaces and had a number equal to the atomic mass. On the basis of this idea he estimated that the solid matter in the atom was about one billionth of the whole atom. This work was influential in hendrik antoon lorentz’s formulation of his theory of electrons and, 10 years later, in ernest rutherford’s proposal of his planetary model of the atom, which incorporates this basic structure.

From 1902 onward, Lenard studied photoelectricity and worked on extending Hertz’s research on the photoelectric effect. In the latter experiments he was able to show that negative electricity can be released from metals by exposure to ultraviolet light. He later found that this electricity was identical in properties to cathode rays (later known as electrons). He also was able to show experimentally that the number of electrons projected is proportional to the energy carried by the incident light, whereas the electron speed or kinetic energy varies inversely with the wavelength of the incident light while remaining independent of its energy. However, it was Einstein who was able to explain the photoelectric effect successfully, in 1905, using the quantum theory rejected by Lenard, by postulating that light consists of particles of energy, which he called photons.

Lenard married Katharina Schlehner and between 1907 and 1931 settled in Heidelberg, as professor of experimental physics. In 1924, he became a dedicated follower of Hitler’s National Socialist Party, which rewarded his zeal by making him the “Chief of Aryan [or German] Physics.” He spent his later years reviling Jewish scientists such as Einstein, whom he never forgave for discovering and giving his name to the theory of light quanta, which explained Lenard’s results.

Antagonistic by nature, Lenard in his book Great Men of Science (1934) omitted such figures as wilhelm conrad rontgen, who had used a tube that Lenard had designed to discover X rays but had failed to credit him. Lenard felt underrated by the physics community, despite his Nobel Prize and a great many lesser honors. He died in Messelhausen, Germany, on May 20, 1947.

Lenard left a mixed legacy to physics. He made substantial contributions to atomic physics but will also be remembered as one of the very few distinguished physicists to embrace Nazism.

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