Optics

OPTICS (from Greek opsis - vision), the doctrine of light, a component part of physics. O. enters partly into the domain of geophysics (atmospheric O., optics of the seas, etc.), and partly into the domain of physiology (physiol. O.). According to its basic physical content, O. is divided into physical O. - the doctrine of the nature of light phenomena - and geometrical O. - the doctrine of light rays, i.e., of the rectilinear propagation, reflection, and refraction of light. To this second department of O. is also attached practical O., i.e., the doctrine of optical instruments based on the properties of light rays and having as their main purpose the improvement of vision (eyeglasses, magnifying glass, microscope, telescope, etc.; to such instruments belongs the photographic apparatus and other instruments). However, the construction of optical instruments is based not only on the data of geometrical O., but also on physical O. Initial knowledge from geometrical O. was already known to the ancients, in particular - plane and spherical mirrors. Optical instruments based on the refraction of light in lenses are mentioned in Arabic literature of the 12th century. The speed of propagation of light was first determined by Roemer in 1675. The first attempts at a scientific explanation of the nature of light belong to Huygens (1690) and Newton (1704). The theory of Huygens considers light as a wave-like motion of an elastic medium - ether, the theory of Newton - as a stream of flying particles. The wave theory of light received general recognition only in the first half of the 19th century. In 1864 it was modified by Maxwell, who put forward the hypothesis that light represents waves not in an elastic medium, but waves propagating in space of an electric and magnetic field (the electromagnetic theory of light). Maxwell's theory received confirmation in the experiments of Hertz (1888) and in many other experimental studies. Further experimental data led to the theory of Planck (1911), that light energy must be emitted and absorbed in certain portions - quanta. Research by Einstein and others led to the view that light quanta move in space as material particles possessing a certain mass, i.e., similar to light particles in Newton's theory. Attempts to unite the wave and quantum theory of light belong to J. J. Thomson and de Broglie. The development of the latter theory led at the present time to Schrödinger to the construction of "wave mechanics" (see Mechanics - wave mechanics), which unites optical and mechanical phenomena. The domain of O. includes not only light in the narrow sense of the word, but also other radiations, the nature of which is identical with the nature of light, but which differ from the latter in wavelength. These include ultraviolet rays, X-rays, gamma-rays of radioactive substances, the so-called cosmic radiation; all these types of rays have a shorter wavelength than light. To this group also belong rays with a long wavelength - infrared rays, as well as electromagnetic waves emitted by electrical apparatus (e.g., radio waves).-In accordance with the above, modern O. is divided into wave and quantum O. To the first department belong phenomena connected with the propagation of light; interference and diffraction of light, as well as reflection and refraction of light waves. To this group also belong the phenomena of polarization of light (see Light) and the diverse phenomena connected with them that occur when light passes through crystals (crystal optics). To the second department belong the emission and absorption of light energy, spectra of visible and invisible light, the photoelectric effect and many other phenomena. This last group of phenomena is closely connected with the question of the structure of matter, of the structure of atoms and molecules, the electronic theory, etc. For medicine, physiol. O. is of great importance, which studies light phenomena in the process of human vision. In addition, medicine plays a large role in a series of optical instruments, starting with the microscope (see) and ending with various instruments used for special purposes, as for example the ophthalmoscope (see Ophthalmoscope, ophthalmoscopy), esophagoscope (see Esophagoscopy), cystoscope (see), rectoscope (see Proctoscopy) and others. Among other medical applications of O. should be mentioned spectral analysis (examination of blood, urine, etc.), as well as polarimetry, i.e., the quantitative analysis of organic compounds with the help of polarized light. This also includes colorimetry, i.e., the method of determining the concentration of solutions by the absorption of light (measurement of the amount of Hb and other substances in the blood). In creating these optical instruments, it was necessary to carry out enormous theoretical work. In particular, Abbe, when designing microscopes with high magnifications, studied in detail the phenomena of diffraction (see separate table), which cause the coloring of the objects under consideration in spectral colors or give a false representation of their structure (see Microscope - art. 226-236). See also Light, Illumination, individual optical instruments.