Philipp Eduard Anton von Lenard, in Hungarian Fülöp Lénárd (born in Bratislava on June 7, 1862 – died May 20, 1947 in Messelhausen) was a physicist and the winner of the Nobel Prize for Physics in 1905 for his research on cathode rays and the discovery of many of their properties.

Philipp Lenard, c. 1900

Biography

Lenard studied under the illustrious Bunsen and Helmholtz, and obtained his doctoral degree in 1886 at the University of Heidelberg. After posts at Aachen, Bonn, Aix-la-Chappell, Breslau, Heidelberg (1896-1898), and Kiel (1898-1907), he returned finally to the University of Heidelberg in 1907 as the head of the Philipp Lenard Institute.

Lenard is best remembered as an "experimentalist of genius" whose major contributions were in the study of cathode rays. Prior to his work, cathode rays were produced in primitive tubes which are partially evacuated glass tubes that have metallic electrodes in them, across which a high voltage can be placed.

Sometime in the 1930's, he joined the National Socialist Party, which made him "Chief of Aryan or German Physics." He, along with Johannes Stark, became something of a fringe group. He was expelled from his post at the University of Heidelberg in the Allied denazification program in 1945. He died two years later.

Photoelectric investigation

The radiant energy was difficult to study because it was inside sealed glass tubes, difficult to access, and because the rays were in the presence of air molecules (fully evacuated tubes didn't produce rays). Lenard overcame these problems by devising a method of making small metallic windows in the glass that were thick enough to be able to withstand the pressure differences, but thin enough to allow passage of the rays. Having made a window for the rays, he could pass them out into the laboratory, or, alternatively, into another chamber that was completely evacuated. He was able to conveniently detect the rays and measure their intensity by means of paper sheets coated with phosphorescent materials.

As a result of his Crookes tube investigations, he showed that the rays produced by radiating metals in a vacuum with ultraviolet light were similar in many respects to cathode rays. His most important observations were that the energy of the rays was independent of the light intensity, but was greater for shorter wavelengths of light.

Another observation that Lenard made was that the absorption of the rays was, to first order, proportional to the density of the material they were made to pass through. This appeared to contradict the idea that they were some sort of electromagnetic radiation. He also showed that the rays could pass through some inches of air of a normal density, and appeared to be scattered by it, implying that they must be particles that were even smaller than the molecules in air. He confirmed some of J.J. Thomson's work, which ultimately arrived at the understanding that cathode rays were streams of energetic electrons.

These observations were explained by Albert Einstein as a quantum effect. This theory predicted that the a plot of the cathode ray energy verses the frequency would be a straight line with a slope equal to Planck's constant, h. This was shown to be the case some years later. The photo-electric quantum theory was the work cited when Einstein was awarded the Nobel Prize in Physics. This much embittered Lenard, who became a prominent skeptic of relativity and of Einstein's theories generally. Ironically, Einstein never really accepted quantum mechanics, and was its most prominent critic.

Links

about Philipp Eduard Anton von Lenard

Nobel Foundation biography

On Cathode Rays

Books by Philipp Lenard

Lenard, Philipp, Great Men of Science. Translated from the second German edition, G. Bell and sons, London (1950) ISBN 083691614X