Molecular Optics

the branch of optics that studies the processes of interaction of optical radiation with matter that depend significantly on the atomic and molecular structure of the substance involved. Molecular optics establishes the relation between the character of single events of interaction of a light wave with particles (molecules, atoms, or ions) and the macroscopic parameters of the medium made up by the particles (such as its index of refraction). Dispersion, refraction, and—most widely—scattering of light are considered in molecular optics from this point of view. The study of propagation of light in crystals that have natural optical anisotropy constitutes the subject of crystal optics. Optical anisotropy in naturally isotropic media may be due to the action of various external fields, such as an electric or magnetic field or a field of mechanical or hydro-dynamic forces. Rotation of the plane of polarization of light takes place in media characterized by optical activity (both natural activity and activity that arises upon application of an external magnetic field). All the phenomena considered in molecular optics provide valuable information on the properties of substances and the structure of their constituent particles.

The process of interaction of a light wave with the particles of a substance is determined primarily by the polarizability of the particles. Classical electron theory has already given an explanation of most optical molecular phenomena, but quantum mechanics, which makes it possible to relate optical molecular constants to the values of the energy levels of molecules and to the probabilities of quantum transitions between levels, is necessary for a complete theoretical interpretation.

The applications of molecular optics are diverse and were broadened by the appearance of sources of high-power coherent radiation (lasers). The methods of molecular optics are most widely used to study the structure and characteristics of individual molecules. The study of light scattered by various media gives information, which is often unique, about the structure of the media—liquids, crystals, macromolecular compounds, and atmospheric formations (such as clouds and fog)—and the peculiarities of the thermal motion of particles in media. Molecular optics is closely related to molecular spectroscopy. Promising molecular-optical methods of studying cosmic bodies and media are being developed.