Optics is the branch of electromagnetism that describes the behavior and properties of light and its interaction with matter (photometry). Optics deals with what are called optical phenomena, on the one hand to explain them and on the other hand to obtain experimental results that allow it to grow as a phenomenological and modeling discipline. There are three branches of optics: geometric optics, physical optics, and quantum optics.
The study of phenomena related to the propagation of light can be done independently of any assumptions about the nature of light radiation. This study, which allows to describe in geometric terms the formation of the image of objects provided by appropriate optical instruments (lenses, mirrors, microscopes, telescopes, etc..), is precisely the subject of the so-called geometric optics. It is based, on the one hand, on the consideration of abstract entities such as light rays, on the other hand, on the principle of rectilinear propagation of light, on the principle of independence of light rays (for which they, although crossing, do not exert any mutual interaction) and on Fermat’s principle, from which it is possible to deduce the laws of reflection and refraction.
The formulas of geometric optics are very simplified if only rays with small inclination on the optical axis are considered (Gauss approximation). Much of the progress in the production of high quality optical instruments is based precisely on the use of these simplified formulas (Gauss optics). The presence of aberrations of predictable magnitude is a direct consequence of the fact that Gauss optics gives only an approximate description of the true behavior of light rays.
However, a large number of optical phenomena cannot be explained without introducing theories about the nature of light. These phenomena are studied in the framework of the so-called physical optics, based on the assumption of a wave-like character of light radiation.
The undulatory model of light interprets exhaustively, in addition to reflection and refraction, also interference, diffraction, polarization, double refraction and a whole series of certain phenomena. However it is not able to explain other phenomena such as emission of atomic spectra and photoelectronic effect. Quantum optics, based on the assumption that light has also a corpuscular nature, is the theory that can better frame all known phenomena related to light radiation. On it are based, in particular, the laser techniques.