Lénárd, Fülöp (Philip)
(Pozsony (Bratislava), June 7th, 1862 – Messelhausen, May 20th, 1947)
He was a Nobel-prize winner physicist, one of the outstanding personalities of the turn of the 19th – 20th century. 
From 1880, he attended the Technische Hochschule in Vienna for one year and, then, he studied at the Budapest University of Science. After a short break, he continued his studies in Heidelberg and obtained his doctorate in 1886. He attended the lectures of Bunsen, Helmholtz, Königsberger and Quincke. Having graduated, he worked as an assistant of Quincke. Then, he also worked in Breslau and Aachen for short time and had the opportunity to get acquainted with the most important physicists of the then Germany.
He achieved his most significant results in Bonn as a colleague of Heinrich Hertz between 1892 and 1894. He was appointed to a professorship at the department of theoretical physics at Heidelberg University in 1896. From 1898, he was a professor at the Kiel University and, from 1907 to his retirement, he taught as a professor at Heidelberg University.
It is in 1888 that he started the examination of cathode rays, which determined his further career. By examining the cathode ray, he demonstrated that the electrons in a cathode ray can be accelerated by means of an electric field and deflected by means of a magnetic field. He recognised that the cathode ray consists of particles of negative charge. These results offered J.J. Thomson the possibility of measuring the specific charge of electrons, which is considered as the discovery of electrons at present. By means of a window made of aluminium foil (Lénárd's window) and placing it on the wall of the cathode ray tube, Lénárd allowed the cathode ray to exit to the atmosphere. By measuring the permeability of the window, he arrived at the conclusion that atoms cannot consist of extensive solids; instead, they are essentially empty.
In 1902, he alone continued the work started with Hertz and recognised that the energy of electrons emerging from metals under the effect of light is proportional to the frequency of light while their number is proportional to the light intensity. He pointed out that, in order for the electrons to emerge, the frequency of light should exceed a specific limit value. His results were reassessed by Einstein, who recognised the quantum-nature of light and developed the photoelectric equation using Lénárd's results.
Late in his life, he actively supported German national socialism. He did not regard two fields of modern physics, namely the theory of relativity and quantum mechanics, as serious natural sciences, primarily due to the origin of the researchers.
Memberships: Associate member (1887) and ordinary member (1907) of the Hungarian Academy of Sciences; honorary doctor of the Universities of Christiania (1911), honorary doctor of Dresden University (1922); honorary doctor of Bratislava University (1942).
Honours: Vienna Academy Baumgartner-Prize of (1896), Royal Society Rumford-Prize (1896), Nobel Prize in physics (1905), Franklin Prize (1905).
References:
Wolff, Stephan L., "Physicists in the 'Krieg der Geister': Wilhelm Wien's 'Proclamation'", Historical Studies in the Physical and Biological Sciences Vol. 33, No. 2 (2003): 337-368.
cathode rays (a short history)
1855 Heinrich Geissler developed the first good vacuum tubes, these tubes, as modified by Sir William Crookes, could produce cathode rays.
1858 Julius Plücker showed that cathode rays bended under the influence of a magnet.
1865 Plücker proved that at lower pressure, the so-called Faraday dark space grows larger in the tube. He also found that there was an extended glow on the walls of the tube and that this glow might be affected by an magnetic field.
1869 J.W. Hittorf found that a solid object put in front of the cathode cut off the glow from the walls of the tube and presented that "rays" from the cathode travelled in straight lines.
1871 C.F. Varley was first to publish suggestion that cathode rays was composed of particles. Crookes meant that they are molecules that have picked up a negative charge from the cathode.
1874 George Johnstone Stoney estimated the charge of these particles to be about 10-20 coulomb, close to the precise value of 1.6021892 x 10-19 coulomb of the electron. He also proposed the name "electrine" for the unit of charge on a hydrogen ion. In 1891, he changed the name to "electron."
1876 Eugen Goldstein presented that the radiation emitted when an electric current is forced through the tube starts at the cathode and introduced the term cathode ray.
1881 Herman Ludwig von Helmholtz showed that the electrical charges in atoms amounted to integral multiple of a smallest unit of electricity.
1886 Eugen Goldstein observed in a cathode-ray tube an other radiation that travels in the opposite direction - away from the anode, later these will be found to be ions.
1892 Heinrich Hertz observed that the rays can penetrate thin foils of metal, and he arrived again at the incorrect conclusion that cathode rays might be some form of wave. Philipp von Lenard ssembled a cathode-ray tube with a thin aluminum window that allowed the rays to leave the tube, such the rays could be studied in the open air. He predicted that cathode rays would move with the velocity of light.
1894 J.J. Thomson published that he has measured the velocity of cathode rays which was much lower than that of light. (1.9 x 107 cm/s, as compared to the value 3.0 x 1010 cm/s for light.)
Einstein's photoelectric equation
hν = Φ + Ek
This equation is an application of the conservation of energy. It states that the energy of a photon (hυ) incident upon the surface of a metal is used partly in freeing the least bound electron (using up energy ν), the remainder being given to the kinetic energy (Ek)of the emitted electron. The new idea was that one electron required one 'quantum' of electromagnetic radiation (above a threshold frequency), to release it. These quanta became known as photons and provided evidence for the particle nature of light.