In the last chapter, we saw the importance of the concept of quantization as a way of solving a large number of problems with which classical physics had struggled without success. Quantization certainly suggested that light—as Einstein had proposed in 1905—is a stream of particles. However, many physicists refused to take quantization as more than an elegant mathematical trick to solve the problems of blackbody radiation, photoelectric effect, and discrete emission lines.
An experiment conducted in 1923 at Washington University in St. Louis by American physicist Arthur Compton finally convinced all but the most die-hard physicists that light is indeed made out of particles. In Compton’s experiment, light (in the form of X-rays) was made to interact with virtually free electrons.
It could be assumed that the electrons were free and at rest, so the solution to the problem shouldn’t be bound by the special cases that needed the “trick” of quantization. Classical physics predicts that the electron should absorb energy from the light wave, and then re-emit the light at the same frequency. However, Compton’s experiments actually showed that light bounces off the electron with lower energy, just as if the light were a stream of particles colliding with the electrons. That is, photons are able to transfer momentum to another particle. That is definitely the signature of a particle! Compton was awarded the 1927 Nobel Prize in Physics for his discovery.