Compton

Arthur Compton received a Doctorate in Physics from Princeton and then spent a year in Cambridge working under Ernest Rutherford investigating the properties of scattered gamma rays. In the early 1920's, at Washington University in St. Louis, he continued this work using X-rays instead of gamma rays.

After several years of work he concluded that the scattering of the X-rays by graphite lowered their energy which meant that the X-rays must be behaving like particles that transferred their energy to the electrons of the graphite in a "collision". This would not happen if X rays behaved exclusively as waves. This provided experimental proof that electromagnetic radiation could exhibit the characteristics of particles as well as waves and the account in the texts makes the most of his confirmation of Einstein's quantum hypothesis. In fact the story is much less clear.

Compton began his career in an atmosphere of virtually universal skepticism toward Einstein's light-quantum hypothesis. His experiments over seven years relied on the work of well known people such as C. G. Barkla, D.C.H. Florence and J. A. Gray. As Compton's X-ray scattering experiments progressed, he rejected one interpretation after another, misread his experimental data, then read it correctly, and in general struggled on his own. Einstein's name does not appear once in Compton's published papers. In the end Compton was nearly scooped in his discovery by Peter Debye, who by contrast was directly influenced by his knowledge of Einstein's light-quantum hypothesis.

Whatever the truth of the matter; Compton scattering confirms the wave particle duality of electromagnetic radiation.