Philipp Eduard Anton von Lenard (German: [ˈfɪlɪp ˈleːnaʁt] ⓘ; Hungarian: Lénárd Fülöp Eduárd Antal; 7 June 1862 – 20 May 1947) was a Hungarian-German physicist who received the Nobel Prize in Physics in 1905 "for his work on cathode rays" and the discovery of many of their properties.

The young Lenard studied at the Pozsonyi Királyi Katolikus Főgymnasium (today Gamča), and as he records in his autobiography, this made a big impression on him (especially the personality of his teacher, Virgil Klatt).

In Heidelberg, he studied under the illustrious Robert Bunsen, interrupted by one semester in Berlin with Hermann von Helmholtz.

Prior to his work, cathode rays were produced in primitive, partially evacuated glass tubes that had metallic electrodes in them, across which a high voltage could be placed.

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.

He was able to conveniently detect the rays and measure their intensity by means of paper sheets coated with phosphorescent and materials.

[10] Lenard observed that the absorption of cathode rays was, to first order, proportional to the density of the material they were made to pass through.

He confirmed some of J. J. Thomson's work, which eventually arrived at the understanding that cathode rays were streams of negatively charged energetic particles.

This theory predicted that the plot of the cathode ray energy versus the frequency would be a straight line with a slope equal to the Planck constant, h. This was shown to be the case some years later.

[16] Lenard is remembered today as a strong German nationalist who despised "English physics", which he considered to have stolen its ideas from Germany.

[17][18][19] Lenard and fellow experimental physicist Johannes Stark were increasingly sidelined and ignored in the 1920's due to their rejection of the theory of relativity and of quantum mechanics.

[20] During the Nazi regime, he was the outspoken proponent of the idea that Germany should rely on "Deutsche Physik" and ignore what he considered the fallacious and deliberately misleading ideas of "Jewish physics", by which he meant chiefly the theories of Albert Einstein, including "the Jewish fraud" of relativity (see also criticism of the theory of relativity).

The publisher included what now appears to be a remarkable understatement on page xix of the 1954 English edition: "While Professor Lenard's studies of the men of science who preceded him showed not only profound knowledge but also admirable balance, when it came to men of his own time he was apt to let his own strong views on contemporary matters sway his judgment.