Optics is for branch of physics dealing with laws of light emission, absorption, and propagation. In physics, the term "light" is used not only for radiation perceived by the human eye, but also for invisible radiation. The nature of light is two-old (dual). It means that light exibits the traits of both an electromagnetic wave and a flow of particles - photons. The dualism of light, in particular, is expressed in formula е = hн because photon energy е is a quantum characteristic, and oscillation frequency н is a characteristic of a wave process. In some optical phenomena, light acted as a wave, and in others, as a particle. The dual nature is inherent to particles as well: electrons, protons, etc. Since light has electromagnetic properties, it is expedient to study optics after electrodynamics. Problems of light emission are on borderline with atomic physics and are essentially connected with it. That is why, the section Optics precedes one on atomic physics.
In the development of physics, optical experiments and theories were of a special importance: laws of rectilinear light propagation and its reflection from mirror surfaces were one of the first physical laws known long before Christ; Michelson's interference experiment provided experimental evidence of the theory of relativity; Planck's hypothesis about special-energy distribution of radiation was the beginning of quantum physics. Studies of visible light and its measurement are not only the province of physics, but also of physiology. In this respect, optics is like acoustics. For physicians and biologists, it is first important for studying biological objects: microscopy, spectrometry, refractometry, polarimetry, calorimetry. In addition, physicians should know the physical principles of thermal radiation in diagnostics of diseases (thermo-graphy), the structure of equipment for light therapy, and other important issues.