Compton’s academic career was largely devoted to the study of x-rays and their interaction with matter. X-rays had been discovered in 1895 by William Röntgen while passing an electrical current through an evacuated glass Crookes tube. The tube, he surmised, was emitting invisible rays which could penetrate a heavy black cardboard shield and cause a distant screen to glow with a fluorescent light. Almost immediately, the usefulness of these new highly-penetrating rays was recognized, and they were soon employed for medical diagnostic purposes. In fact, the earliest x-ray images are of Röntgen’s wife’s hand—revealing her skeleton beneath a thin shadow of flesh. The true nature of x-rays, however, remained a mystery until 1912, when Max von Laue discovered that they exhibit a diffraction pattern when passed through a copper sulfate crystal. This confirmed that the mysterious x-rays were indeed electromagnetic waves with lengths comparable to the atomic spacing of the crystal. Inspired by von Laue’s work, William Henry Bragg and his son, William Lawrence Bragg, carried out a systematic investigation of crystal structures using the new technique of x-ray diffraction.
But are these x-rays truly waves? Since Einstein’s 1905 publication, theoretical and experimental evidence was beginning to suggest that light behaves—at least in certain situations—like discrete packets of energy. Could this quantum theory of light be extended to x-rays? And if so, how could it be experimentally verified? These are the issues which Compton addresses in the reading selection contained in the next two chapters.