the appearance of a mechanical torque acting on a body illuminated by elliptically or circularly polarized light. The phenomenon was theoretically predicted by the Russian scientist A. I. Sadovskii in 1898. The Sadovskii effect is a consequence of the nonzero angular momentum present in an elliptically polarized electromagnetic wave; the momentum is transferred by the wave to the illuminated body, which either absorbs the wave or changes the state of polarization.
The Sadovskii effect is very small; nevertheless, it has been detected experimentally for both visible light and centimeter waves. The first successful experiments were performed by the American scientist R. Beth in 1935. The theoretical evidence for the existence of the Sadovskii effect was an indication that the law of conservation of angular momentum can be applied to the interaction between electromagnetic radiation and matter; this hypothesis later became an integral part of the quantum theory for matter-radiation interactions. As a result, it became possible to describe many details of the processes by which atoms and molecules emit and absorb light and to predict and confirm other effects (see, for example, OPTICAL ORIENTATION).
From a quantum-mechanical standpoint, the presence of angular momentum in a stream of photons (the quantum analog of a light wave) is correlated with the observation that the orientations of photon spin and angular momentum along the direction of photon travel and in the opposite direction occur with unequal probabilities for the case of elliptical polarization. For circular polarization, which is a limiting case of elliptical polarization, the orientation of photon angular momentum can be described with certainty; its absolute magnitude is equal to Planck’s constant.