light in the broad sense of the word, electromagnetic waves with wavelengths between 1 nanometer and 1 millimeter. Infrared radiation and ultraviolet radiation, as well as the visible radiation perceived by the human eye, are classified as optical radiation. The term “light” is close in meaning to “optical radiation” but historically has had less definite spectral boundaries—often not all optical radiation but just the visible band has been called light. The forming of directed radiation fluxes by means of optical systems incorporating such devices as lenses, mirrors, optical prisms, and diffraction gratings is characteristic of optical methods of research.
The wave properties of optical radiation are responsible for such phenomena as diffraction, interference, and polarization of light. A number of optical phenomena, however, cannot be understood unless optical radiation is conceived of as a flux of fast particles, or photons. This wave-particle duality of optical radiation connects it with other objects in the microworld and finds a general explanation in quantum mechanics. The speed of light, or the propagation velocity of optical radiation, in a vacuum is about 3 × 108 m/sec. In other mediums the speed of optical radiation is lower. The value of the refractive index of a medium is given by the ratio of the speeds in a vacuum and in the medium and is in general different for optical radiation of different wavelengths. The dispersion of optical radiation is a result of this fact.
The types of optical radiation are classified according to such features as origin (thermal radiation, luminescent radiation), homogeneity of spectral composition (monochromatic, polychromatic), orientation of electric and magnetic vectors (natural, linearly polarized, circularly polarized, elliptically polarized), and degree of diffuseness of the radiation flux (directional, diffuse, mixed).
The optical radiation flux incident on the surface of a body is partially reflected, partially transmitted through the body, and partially absorbed by the body. The absorbed part of the optical radiation energy is converted primarily into heat and increases the temperature of the body. Other types of conversion of the energy of optical radiation, however, are also possible. Examples are the photoelectric effect (photoelectric emission), photo-luminescence, and photochemical conversions.
For the role of optical radiation and optical methods of investigation in science and technology see and its bibliography,
IU. S. CHERNIAEV