Despite the popularity of Einstein's theories of relativity and his musings on black holes, Einstein's Nobel Prize in physics was actually awarded for his discovery of the photoelectric effect. This discovery revolutionized our understanding of the world around us. But what is the photoelectric effect?
A few more steps brought physicists closer to the answer, including JJ Thompson’s identification of the emitted particles as electrons. The next big breakthrough came from Philipp Lenard who discovered that changing the intensity of the incident light had no effect on the energy of the emitted electrons. Doubling the intensity doubled the number of electrons that were produced, but had no effect on their energies.
Lenard’s observation directly contradicted predictions based on our understanding of light as a wave. As a wave, brighter light was expected to shake the electrons more violently, and thus dislodge more electrons and at faster speeds. Lenard further observed that there was a well-defined minimum threshold energy for the incident light, below which no electrons were released at all.
The existence of such a minimum was also at odds with the wave description of light. Even as advancements in our understanding of the details of the photoelectric effect continued, there were still few answers as to why the observations did not match the theory.
Wave Particle Duality of Light
In 1905, Einstein reported that all of the observed phenomena could be explained if light was thought of as a stream of particles (or quanta of light called photons) rather than as a wave. These photons each have an associated energy equal to the frequency of the light multiplied by a constant. In other words, the energy of each photon is proportionate to the frequency of the light.
In the metal slab experiment, each photon can be imagined as a particle that hits a single electron and dislodges it from the metal. Some energy is lost in that process, so the resulting electron has the net energy of the incident photon less whatever energy was needed to free it. Thus, the energy of the produced photoelectrons will vary with the frequency of the incident light, but not with the intensity. Instead, the intensity (i.e. how many photons hit the metal) will only affect the number of photoelectrons produced.
Einstein’s theory also explains Lerner’s minimum energy value: if the incident light has energy values (i.e.., frequencies) that are lower than the energy required to free an electron from the metal, the electrons stay put.
The American experimental physicist Robert Millikan was not ready to accept Einstein’s theory and to do what he saw as abandoning the wave theory of light. He spent ten years trying to disprove Einstein, but only ended up instead proving repeatedly that his discovery of the photoelectric effect appeared to be correct. Luckily for Millikan, he still received a Nobel Prize for his results.
Where Would We Be Without the Photoelectric Effect?
The photoelectric effect has direct applications in the use of photocells and solar cells where energy is produced due to incident photons. More importantly, however, the photoelectric effect set off the quantum revolution. Experimental physicists began to think about the nature of light and the structure of atoms, the foundation of the world around us, in an entirely new way.
Perhaps the biggest lesson to be learned from Einstein’s work on the photoelectric effect is to always remember to think outside the box. If our normal theories aren’t working, sometimes the answer is to make new ones. Einstein himself said, “We cannot solve our problems with the same thinking we used when we created them.”
Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science.
If you’re interested in learning more about Einstein and the theory of relativity in honor of its 100th birthday, check out this month’s Scientific American magazine special edition at www.scientificamerican.com.
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