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Einstein’s Legacy: The Photoelectric Effect

Photoelectric Effect

Photoelectric Effect

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

The law of the Photoelectric Effect

When you think of Albert Einstein, what do you think of? General relativity? Black holes? Crazy hair? While he certainly made significant contributions to all of those topics during his lifetime, Albert Einstein was perhaps even more well known in his time for his work to understand the photoelectric effect.

In fact, when he was awarded the Nobel Prize in Physics in 1921, the honor was stated to be “for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect.”

This discovery is so important — and Nobel Prize worthy — because Einstein suggested for the first time that light is both a wave and a particle. This phenomenon, known as the wave-particle duality of light, is fundamental to all of quantum mechanics and has influenced the development of electron microscopes and solar cells.

What is the Photoelectric Effect?

When light with energy above a certain threshold hits a metal surface, an electron that was previously bound to the metal is knocked loose. Each particle of light, called a photon, collides with an electron and uses some of its energy to dislodge it from the metal. The rest of the photon’s energy is transferred to the now free-roaming negative charge, called a photo-electron.

So why does this happen? What determines the energies (and speeds) of the emitted electrons? To understand the answers to these questions, we need to dig a little into the history of the discovery of the photoelectric effect.

James Clerk Maxwell

Photoelectric Effect James Clerk Maxwell
James Clerk Maxwell

In the late 1800’s, experimental physicists had a huge task in front of them. In 1865, the mathematical physicist James Clerk Maxwell published his theory of electromagnetism in which he claimed electricity and magnetism both moved through space as waves traveling at the speed of light.

Experimentalists then set out to find observational evidence of Maxwell’s theories which so nicely explained the properties of light — at least mathematically.

Heinrich Hertz

Photoelectric Effect Heinrich Hertz
Heinrich Hertz

Success came in 1887, when Heinrich Hertz was the first to generate and detect the electromagnetic radiation predicted by Maxwell. Hertz created a spark between two pieces of brass using a high voltage induction coil, and then was able to detect the resultant radiation from that spark when its oscillations created a second spark between a copper wire and a brass sphere (his “receiver”) up to 50 feet away.

This second spark was very faint, however, so to get a better view of it, Hertz tried enclosing his receiver in a dark box. Unexpectedly, he found the enclosure diminished the receiver’s spark, but only when the box was made of certain materials. After what was probably months of investigation, Hertz concluded that the best way to increase the receiver’s spark was to use ultraviolet light (i.e. light at a higher frequency than optical light).

However, at the end of his investigation, Hertz still did not know why such an effect occurred. He stated, “I confine myself at present to communicating the results obtained, without attempting any theory respecting the manner in which the observed phenomena are brought about.”

Philipp Lenard

Photoelectric Effect Philipp Lenard
Philipp Lenard

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 article duality of light

Wave article 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.

Robert Millikan

Photoelectric Effect Robert Millikan
Robert Millikan

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.”


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