Light knocks Physics for a leap — The Photo-electric Effect

In 1901, German physicist Phillipp Lenard discovered light's curious interaction with metal but could not explain it.

In 1901, German physicist Phillipp Lenard discovered light’s curious interaction with metal but could not explain it.

By the turn of the 20th century, physics seemed at a standstill.  “The more important fundamental laws and facts of physical science have all been discovered,” an American physicist lamented.  But the century was barely a year old when light shook physics, science, and certainty itself to their roots.

Late in 1901, in a small laboratory in the German port city of Kiel, physicist Philipp Lenard hooked two metal plates to batteries and placed them in a vacuum tube.  Lenard had been studying such “cathode tubes” for more than a dozen years.  He knew that by increasing the voltage through the plates, he could cause a spark of “cathode rays” to jump between them.  But what else might trigger such a spark?  Other scientists had seen light create cathode rays that caused glass tubes to glow.  Now Lenard set out to study precisely how light performed such a feat.

Shining an ultra-violet beam on his plates, Lenard saw the expected spark.  Without any added voltage, any other stimulus, light caused electrons to flow.   Somehow, the bearded, bug-eyed Lenard concluded, light knocks electrons off metal.  But the plot thickened when Lenard measured the electrons’ charge.

Common sense and the prevailing wave theory of light suggested that brighter light should release more highly charged electrons.  And there should be an accumulation of energy, a dim light shining for seconds before finally gathering enough energy to knock off an electron.  But light refused to cooperate with common sense or prevailing theory.  Instead, Lenard discovered that below a certain brightness, light displaced no electrons, no matter how long it shone.  Even more surprising was the consistency of electrons released.  No matter how bright the light that hit Lenard’s plates — even a blinding arc lamp — all the freed electrons had the same charge. The only way to boost the charge was to use light of a different color. Tightly-wound ultra-violet waves freed stronger electrons than longer-waved blue.  But if light is made of waves, how could it knock off individual electrons?  Only particles of light could do that, and nearly a century had passed since anyone thought light to be made of particles.  Particle or wave?  The mystery intrigued physicists throughout Europe.  Then came 1905 and a man named Albert Einstein.

Albert Einstein, in 1905, applied the nascent theory of quantum energy to Lenard's discovery.  Einstein's explanation of the photoelectric effect won the Nobel Prize for physics and unleashed the quantum revolution.

Albert Einstein, in 1905, applied the nascent theory of quantum energy to Lenard’s discovery. Einstein’s explanation of the photoelectric effect won the Nobel Prize for physics and unleashed the quantum revolution.

When he read of Lenard’s discovery, Einstein was so desperate for work that he was  considering playing his violin on the street.  He had finished his Ph.D. thesis, yet it had not been accepted.  Though out of work, he remained fascinated by the latest developments in physics.  “I have just read a wonderful paper by Lenard,” Einstein wrote his future wife and fellow physicist Mileva Maric.  “Under this beautiful piece, I am filled with such happiness and joy that I absolutely must share some of it with you.”  Three years would pass before Einstein solved the mystery posed by Lenard.  The solution would change everything.  Everything.

Einstein wrote four papers in 1905.  One dealt with the relativity of space and time, another linked energy and matter.  A third was on “Brownian motion,” the strange wiggle of particles in a fluid.  Yet the one he called “very revolutionary” explained light and its “photoelectric effect.”  Why did light striking metal free electrons of a specific charge?  The answer came from another revolution in the making.  Back in 1900, German physicist Max Planck had proposed that energy came not in particles or waves but in packets.  As energy increased, as light brightened or heat rose, its accumulation was not uniform but came in predictable jumps.  Like a man climbing a ladder, energy emissions could not stand between rungs.  Instead, they leapt from one level to the next as multiples of a constant named after Planck himself.  Planck called the packets “quanta,” after the Latin word for “how much.”   Reading of Planck’s work, Einstein recalled, “It was as if the ground had been pulled out from under us, with no firm foundation to be seen anywhere.”  By 1905, he was building a new foundation.

While working at the patent office in Bern, Switzerland, Einstein solved Lenard’s mystery by applying Planck’s constant. The constant was incredibly small — a decimal preceded by two dozen zeros — yet when Einstein applied it to Lenard’s measurements, the numbers added up.  Higher frequencies of light — blue or ultra-violet — come in larger energy packets than lower frequencies.  “According to the assumption to be considered here,” Einstein wrote, “when a light ray is propagated from a point, the energy is not continuously distributed over an increasing space but consists of a finite number of energy quanta which are localized at points in space and which can be produced and absorbed only as complete units.”  Only if light were not waves but quanta — Lichtquanten in Einstein’s German — could it displace individual electrons.  As to why different colored light released electrons of a different charge, Einstein noted the amounts of energy in each hue. “The simplest conception is that a light quantum transfers its entire energy to a single electron.”

Throw rocks at a cliff and you knock off dirt clods. Throw harder and you knock off bigger clods — a simple, Newtonian transfer of energy. But as of 1905, light no longer belonged to Newton’s universe.  In Einstein’s universe, light striking metal knocks off electrons according to the size of the particles thrown, not how hard they are thrown.  Bigger rocks alone knock off bigger clods. Waves would wash away entire sections of the cliff.  Only rock particles — quanta — can pick off individual electrons.

Another decade would pass before Einstein’s Lichtquanten would be proven.  Even Max Planck objected, saying that quanta did not apply to visible light, which everyone, even Einstein, thought to be a wave.  Then in 1915, American physicist Robert Millikan confirmed both Planck’s constant and Einstein’s photoelectric theory.  “I spent ten years of my life testing that 1905 equation of Einstein’s,” Millikan said, “and contrary to all my expectations, I was compelled to assert its unambiguous verification in spite of its unreasonableness, since it seemed to violate everything we knew about the interference of light.”

By 1920, light’s quirky behavior had overcome decades of skepticism.  The photo-electric effect had proved the existence of light quanta.  Using light to spark electricity would become the basis of TV cameras, light meters and many other devices.  More importantly, Einstein’s use of quanta prepared physics for its quantum leap.  In 1921, Einstein won the Nobel Prize for physics, not for relativity but for the photo-electric effect.  Yet to the end of his life, he would wonder about light.  “All these fifty years of pondering have not brought me closer to answering the question, What are light quanta?”


Author photoBruce Watson is the author of Light: A Radiant History from Creation to the Quantum Age (Bloomsbury, February 2016).  The book traces humanity’s evolving understanding and control of light, starting with creation myths, then moving into scripture, philosophy, architecture, Islamic science, art history, poetry, physics, and quantum physics.

Watson’s previous books include Freedom Summer, Sacco and Vanzetti, and Bread and Roses.  Watson’s work has appeared in the Boston Globe, the Los Angeles Times, American Heritage, the Wall Street Journal, the Washington Post, Yankee, Reader’s Digest, and Best American Science and Nature Writing 2003.

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