Although Einstein managed to chalk up several successes for his photon concept, he failed to come up with the slam-dunk needed to earn universal acceptance. Such proof eventually came from an important experiment that relied, believe it or not, on Newton’s laws of motion. (Truth be told, it required just a smidge of Einstein’s theory of special relativity, too.)
The experiment was performed in 1923 at Washington University in St. Louis. There, Arthur Compton discovered that when high frequency light was directed onto a carbon sample, some of the light that scattered off to the side experienced a shift in frequency. The larger the scattering angle, the larger the shift. This could not be explained by Maxwell’s wave theory.
From the classical viewpoint, when a light wave interacts with one of the carbon atoms, it would set an electron into oscillatory motion at the same frequency of the incoming light. In turn, the oscillating electron would emit a secondary electromagnetic wave also at the same frequency. Maxwell’s equations therefore predict no shift in frequency, no matter what direction the secondary (scattered) waves travel.
QUANTUM LEAP
It seems only fitting that Compton drew on Newton’s laws to validate the particle nature of light—and at the expense of Maxwell’s wave theory. After all, it was Maxwell who had earlier extinguished the last vestige of hope for Newton’s light corpuscles.
Given the inability of wave theory to explain the experiment, Compton decided to give Einstein’s particle theory a shot. He treated the incoming photon like a quantum cue ball and then calculated the frequency shift you’d expect if it collided with one of the electrons in his sample. When he turned the mathematical crank and compared his prediction to measurement, the particle interpretation turned out to be nearly spot on.
This schematic illustrates the frequency shift observed by Compton (left). The relatively good agreement between the quantum theoretical curve and the measurement frequency shift data provided strong evidence that light was composed of particles (right).
Aside from a small bit of special relativity, Compton’s calculation relied simply on Newton’s laws of motion and the concept of energy conservation. Notably, his derivation was hardly more difficult than if he’d calculated the outcome of a Bocce ball collision. Nevertheless, his discovery silenced the remaining naysayers and gave the photon a permanent spot within the lexicon of physics.
It’s worth noting that Compton used X-rays, a very high-frequency variety of electromagnetic radiation. Had he used visible or even ultraviolet light instead, he would have had a heck of a time measuring the frequency shifts. This is because the relative size of the shift is much less pronounced for lower and lower frequencies. At extremely low frequencies, it turns out that the experiments approach the classical wave prediction of no frequency shift at all.
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