In the first decades of the twentieth century, scientists increasingly realized that the objects of the microcosm possess simultaneously the properties of both particles and waves (cm. The principle of complementarity). This process was initiated by Albert Einstein’s explanation of the photoelectric effect, according to which any electromagnetic radiation, including light, is a beam of photons. The effect of scattering of photons by free electrons, discovered by the American physicist Arthur Compton, became another confirmation of the quantum nature of the photon.
Compton’s experiment is easy to describe. A beam of electromagnetic rays (Compton used X-rays) is directed at the crystal, after which the energies and the angle of deflection of the scattered rays are measured. Within the framework of the classical theory of the interaction of rays with matter (before postulating the principles of quantum mechanics), the energy of the reflected radiation should not differ from the energy of the initial radiation. Compton got a fundamentally different picture: the energy of the scattered wave differed from the energy of the original wave, and this difference depended on the scattering angle, reaching a maximum at an angle of 90 °. The only way to give a reasonable interpretation of the results obtained by Compton was to consider the interaction of rays with atoms as a collision of an outgoing particle (photon) with an electron. Like two billiard balls, these two particles interact and bounce off each other. And since the electron moves slowly, in the general case, it must acquire energy in this collision, while the photon loses the same energy.
After the publication of the obtained results by Compton in early 1923, there were few physicists who doubted the reality of photons. Today the Compton effect finds application in astrophysics: gamma rays from space objects are repeatedly scattered until their energy falls to the wavelengths of the X-ray part of the spectrum, after which they can be analyzed on standard X-ray equipment. A similar detector was put into orbit by NASA in 1991 as part of the Compton Gamma Ray Observatory.