The Origin of The Photoelectric Effect

When we talk about the photoelectric effect, we usually only mention Albert Einstein.  After all, Einstein did win the Nobel Prize for creating the modern interpretation of the photoelectric effect.  But Einstein didn’t discover the photoelectric effect.  In fact, Einstein didn’t do any photoelectric effect experiments!  Instead, Einstein was inspired by the experiments and theories of a man named Philip Lenard.  Lenard is a fascinating person whose So who did discover it, who did the landmark experiments, and why did they do them?  This is the story of Heinrich Hertz’s discovery of radio waves and his assistant named Phillip Lenard who kept on being almost at the forefront of new discoveries until his disappointments and mental weakness drove him to Nazism.  Ready?  Let’s go!

Let’s start around 1880 when

Lenard first heard about something called cathode rays when he was a teenager in 1880 or so. 

Let’s start in a dark laboratory in Germany in May of 1887.   That was where a 30-year-old scientist named Heinrich Hertz was experimenting with the large sparks from an induction coil.  He had found that if he added an antenna to the coil he could “catch” invisible waves by seeing little sparks in another coil a distance away.  Hertz thought he was validating Maxwell’s equations (which he was) but he was also discovering the radio wave, which is why frequency is measured in Hertz.  This experiment was not in any way easy, and Hertz recalled that “the sparks are microscopically short, scarcely a hundredth of a millimeter long.  They only last about a millionth of a second.  It almost seems absurd and impossible that they should be visible.[1]”  Because the spark in the receiver was so small he decided to put it in a dark box so the light from the bright spark from the induction coil wouldn’t distract him.  To his surprise, when he did this the spark in the receiver became “decidedly smaller inside the case than it was before.[2]”  He started putting all types of screens between the transmitting spark and the receiving spark.    Hertz found that glass would reduce the spark but quartz did not.  As glass blocks ultraviolet light and quartz do not, Hertz decided that the spark must be amplified in some way by the energy of the ultraviolet light, or at least he decided that “until the contrary is proved.[3]”  Since the ultraviolet light from the first spark caused a bigger electric spark in the receiver, this was eventually called the photoelectric effect (photo for light, electric for, well, um, electric).  In his own words, Hertz didn’t attempt, “any theory respecting the manner in which the observed phenomena are brought about.[4]” 

Philipp Lenard
Philipp Lenard

Hertz didn’t study the photoelectric effect further but he triggered an avalanche of studies into it[5].  One of the many people who had a theory about the photoelectric effect was a 27-year-old grad student named Phillip Lenard.  Lenard had seen that when metal plates were exposed to strong UV light, they stopped looking so smooth.  Lenard had this interesting theory that electricity could only be carried by “dust” not by gasses, and that the UV light was freeing the dust, which caused the plates to look rough.  This theory was roundly ignored by basically everyone and seemed to go nowhere.   However, three years later, Lenard managed to get a job as an assistant for Hertz.  By this time, Hertz was studying cathode rays or rays that travel from a negative electrode in vacuum tubes (Hertz was convinced that they were a type of invisible light, like his radio waves).  In June or July of 1892, Hertz thought he found proof that cathode rays were beams of “ultra-ultra violet light” because he found that they would travel through thin pieces of metal. Hertz thus told Lenard to make a vacuum tube with an aluminum barrier.  According to Lenard himself, Hertz did not talk to him very much, which was probably a sign that Hertz didn’t like Lenard as Hertz was a very friendly man.  In fact, it was possible that Hertz only asked Lenard for help as Hertz was starting to get terrible headaches and feel sick all of the time.  By October, Hertz had several operations and by Christmas, he wrote his parents, “It seems only yesterday that it was the middle of summer, and since then I have not been aware of anything that has happened, or that I have experienced anything but a dreadful dream from which I still cannot awaken.[6]

Meanwhile, left alone in the laboratory, Lenard was busy working away on Hertz’s plan for an aluminum barrier.  Lenard actually took it a step farther and, as well as an aluminum barrier between areas of different vacuums, Lenard also made an aluminum window between the vacuum tube and the air.  Lenard was quite surprised to find (he actually said, “nobody could have predicted this”), that not only could the cathode rays escape the tube they could also travel a few inches in the air and make a phosphorescent screen glow in a darkened room.  Lenard was sure this paper would finally make his name in science and published his results in December of 1893.  Hertz died mere weeks later and Lenard took time off of his research to edit German and English memorials of Hertz’s work and biography.  This was unfortunate timing for a distraction from work as just the next year another German scientist named Wilhelm Roentgen was experimenting with a “Lenard tube” with a heavier phosphorescent screen and discovered a new ray, which Roentgen called an x-ray.  Suddenly, the whole world was in an x-ray craze.  Lenard wrote Roentgen a nice letter that, “because your remarkable discovery caused such remarkable attention in the farthest circles, my modest work has also come into the limelight.[7]”  And at first, Lenard was correct, as both Lenard and Roentgen shared the Rumford medal in England and the Matteucci Medal in Italy.  Lenard finally got an associate professor position, and, after setting up his laboratory decided to get married, as he had a decent job and, “there was an abundance of daughters of professors who were waiting to be married.[8]”  Soon, he was made a full professor and his career (and family life) seemed secure.  However, in 1897, JJ Thomson discovered that the cathode ray wasn’t a type of light at all but was instead a beam of charged particles that we now call a beam of electrons.  With the discovery of the electron, Lenard found that his research in the cathode ray was made moot.  Moreover, if the cathode ray was a ray of particles and the x-ray was a ray of ultra-ultra violet light (which is what we now think), then Lenard’s discovery that cathode rays could travel in the air had very little to do with Roentgen’s discovery of x-rays.  In other words, Lenard thought that Roentgen had just discovered a new type of cathode ray in air.  But really, Roentgen had discovered something new, and Lenard hadn’t.  It was a crushing defeat. 

Photoelectric Effect

Lenard repeated JJ Thomson’s experiment and found that, yep, “in every respect, the rays behaved like moving masses carrying a negative charge.[9]”  Lenard tried to play it cool, as electrons act like moving masses but maybe they were really just, “pieces of the ether… which move individually, possess mass, and seem to be identical to the carriers of electric charge[10]”.   Either way, it seemed like all of Lenard’s discoveries were being usurped by others, and didn’t publish anything for a year and a half.  Lenard then had an ingenious thought, what if his old theory that charges were only carried by “dust” was right if the “dust” was really just electrons or cathode rays.  In October of 1899, Lenard published; “The Generation of Cathode Rays with Ultraviolet Light” where he demonstrated that, yes, ultraviolet light produced electrons.  He proved this in several ways, first by shining ultraviolet light on a sodium plate with a platinum anode in a very very strong vacuum.  Lenard then burned the platinum and proved with spectroscopy that none of the sodium moved from the cathode onto the anode[11].  He then demonstrated by how the beam deflected in a magnetic field that the mass to charge ratio was the same as that of an electron.  He also proved that ultraviolet light could produce electrons in tubes that had too high a vacuum to produce cathode rays.  

In December of 1900, a man named Max Planck published his equations for blackbody radiation where he assumed that light came in little energy elements with energy equal to constant times the frequency of the light.  This would explain why ultraviolet light would produce different effects than visible light as it has a higher frequency and therefore is composed of elements of higher energy.  However, Lenard didn’t see the connection, and frankly neither did Planck.  Meanwhile, Lenard had bigger problems.  In 1888, Alfred Nobel who had invented dynamite became very nervous about his reputation after reading a premature obituary.  He then rewrote his will so that 94% of his fortune went to set up a yearly prize in Physics, Chemistry, Medicine, Literature, and Peace.   Nobel died in 1895, but it took until 1901 to give the first award.  Anyway, the foundation decided to give the first Nobel Prize in Physics to Roentgen for discovering the x-ray without mentioning Lenard.   

Lenard was very upset and pushed that he was unfairly treated.  Meanwhile, he continued to work on the particles created by ultraviolet light in a vacuum.  Lenard found that if he added a voltage in the wrong way he could drive the electrons back to the irradiated plate “in the same way as a stone thrown upwards falls back to the ground.[12]”  Now, he could experimentally add just enough voltage to drive back the electrons and therefore measure the maximum energy of the electrons, which obviously related to the speed of the electrons.  To his surprise, “the initial velocities are independent of the intensity of light.[13]” 

In other words, more intense light made more electrons but did not give the individual electrons more energy.  What does this all mean?  Well, Lenard had some interesting and completely wrong conclusions.  Lenard thought that since the speed of the electrons was independent of the intensity of the light that energy didn’t come from the light at all.  Instead, he thought that the light released the energy inherent in the molecule like, “the fuse in firing a loaded gun.[14]”  This “triggering” theory of atoms seemed like a good solution and even as late as 1909 it was written as one of them, “generally accepted truths in physics[15].”

In 1905 two events happened that totally changed Lenard’s life.  The first was Lenard was awarded the Nobel Prize in physics for his work in cathode rays.  This, despite the fact that the 14 top scientists nominated JJ Thompson for discovering the electron and only 2 scientists nominated Lenard (don’t worry Thomson won it the next year).  In his speech accepting the award, Lenard told the audience that he would, “now speak not only of the fruits but also of the trees which have borne them, and of those who planted these trees.  This approach is more suitable in my case, as I have by no means always been numbered among those who pluck the fruit; I have been repeatedly only one of those who planted or cared for the trees.[16]

The second event was that in June of 1905 a patent clerk named Albert Einstein published a little paper where he proposed looking at many phenomena, including the photoelectric effect if the light comes in little energy packets, that he called quanta.  This was the first use of Plank’s equation that the energy of light is equal to constant times the frequency as a fundamental truth instead of a mathematical trick.  Einstein concluded, “as far as I can see no contradiction exists between our understanding and the properties of photoelectric action observed by Mr. Lenard.”  Lenard (or possibly an assistant) saw how often Einstein referenced him and sent Einstein a copy of his latest paper.  Einstein replied with an enthusiastic letter in return.  It took Lenard 4 years to respond by which time Einstein was a professor but when he did Lenard’s letter was effusive in praise for a low-level assistant physics professor from a Nobel Prize-winning physicist.  Lenard gushed, “what could be more exciting for me than when a profound comprehensive thinker finds favor with some points from my work” and closed his letter, “with excellent regard, your loyal P. Lenard.”  At around this time, Einstein learned that his friend named Jakob Laub had got a job as an assistant to Lenard.  Einstein wrote Laub, “I took great pleasure in this news… I think that the opportunity to work with Lenard is worth far more than the assistantship and income combined… [Lenard] is a great master, an inventive thinker!”  However, Laub’s relationship with Lenard quickly soured and Einstein’s view of Lenard went with it. 

See, Lenard was devoted to the idea of the aether, invisible weightless material that fills the universe even in a vacuum so that light will have something to vibrate (after all, they thought, if you have a wave, you need a wave of something).  In fact, Lenard insisted that electrons were really just bunches of aether.  However, by 1910 this magical material was losing ground.  Lorentz had made complicated equations to describe how aether could cause all sorts of odd contractions but Einstein’s relativity equations explained you could get those odd contractions and you didn’t need aether.  Laub started complaining to Einstein that Lenard forced him to waste his time on experiments to prove the existence of ether.  Einstein wrote Laub “[Lenard’s] recent lecture on these fanciful ethers appears to me almost infantile.  Further, the study he commanded of you… borders on the absurd.  I am sorry that you must spend your time on such stupidity.”   Einstein tried to help Laub get another position, a fact that Lenard found intolerable and earned Lenard’s ire for both Laub and Einstein.  Einstein wrote Laub, “[Lenard] is really a twisted fellow… So entirely composed of gall and intrigue.  However, you are considerably better off than him.  You can get away from him, however, he must do business with the monster until he bites the dust”

Even worse, in 1914, Lenard signed the “manifesto to the civilized world” which included the line “Without German militarism German civilization would be wiped off the face of the earth[17]”.  Not only did Einstein refuse to sign the manifesto, but he also co-wrote a rebuttal.

Lenard decided that the big problem was that Einstein was Jewish, and it was a Jewish conspiracy to get rid of aether, push relativity, quantize the photoelectric effect and discredit him.  Lenard fought as well as he could to keep Einstein from gaining a Nobel Prize.  In fact, when the Nobel committee finally defied Lenard they gave Einstein the prize “, especially for his discovery of the law of the photoelectric effect”. 

Lenard then went on to become a ranking and early member of the Nazi party and the head of “Aryan Science” for Hitler.  He prided in how he drove Einstein and other Jewish scientists out of Germany although he would have preferred their death.  (he once sent an attempted murderer of a Jewish official a letter of congratulations and also said in a speech, “more than ever, the Jews must be sunk right down to the center of the earth![18]”)  However, the Nazi elite eventually started to ignore Lenard as he wasn’t very useful for the war effort.  He survived the war and died in 1947 of old age.  Whew, good riddance.

But let’s go back a bit to Einstein and the Nobel Prize.  Although they probably only gave him the prize for the photoelectric effect as a way to irritate Lenard, his 1905 paper was truly transformative and, as a bonus, still believed to be true well over 100 years later.  How Einstein saw light is next time on the Lightning Tamers.


[1] p 98 “Electric Universe”

[2] Hertz, H “On an effect of Ultra-violet light upon the electric discharge” Translated by Jones, D Electric Waves (1893) p. 63

[3] Hertz, H “On an effect of Ultra-violet light upon the electric discharge” Translated by Jones, D Electric Waves (1893) p. 76

[4] Hertz, H “On an effect of Ultra-violet light upon the electric discharge” Translated by Jones, D Electric Waves (1893) p. 79

[5] Wheaton, B “Philipp Lenard, and the Photoelectric Effect, 1889-1911” Historical Studies in the Physical Sciences Vol. 9 (1978) p. 301

[6] Hertz, H to parents Dec. 23, 1892, in Bodanis, D Electric Universe (2005) p. 107-8

[7] referenced on p. 71 Glasser, Otto Wilhelm Conrad Roentgen and the Early History of the Roentgen Ray 1933,

[8] Lenard, P. translation found in Hilman, B. Ertil-Wagner, B, and Wagner, B The Man Who Stalked Einstein (2015) p. 24

[9] Lenard, P translated by Wheaton, B “Philipp Lenard and the Photoelectric Effect, 1889-1911” Historical Studies in the Physical Sciences Vol. 9 (1978) p. 309

[10] Lenard, P translated by Wheaton, B “Philipp Lenard and the Photoelectric Effect, 1889-1911” Historical Studies in the Physical Sciences Vol. 9 (1978) p. 309

[11] Lenard, P “Generation of Cathode Rays by Ultraviolet Light” Annalen der Physik Vol. 2 (1900) p. 359

[12] Lenard, P “On Cathode Rays” Nobel Lecture (May 28, 1906) p. 122

[13] Lenard, P. “About the Photoelectric Effect” Annalen de Phisik, Bd. 4 (March 1902) p. 150

[14] p 123 “On Cathode Rays” Nobel Prize Speech 1905 Lenard

[15] Ladenburg, R translated by Wheaton, B “Philipp Lenard and the Photoelectric Effect, 1889-1911” Historical Studies in the Physical Sciences Vol. 9 (1978) p. 319

[16] Lenard, P “On Cathode Rays” Nobel Lecture (May 28, 1906) p. 105

[17] “Manifesto to the Civilized World” Translation found in Holmes, Virginia Einstein’s Pacifism and World War 1 (2017) p. 5

[18] Lenard quoted by Hentschel, K Physics and National Socialism (2011) p. 115

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