LIGHT is the subject of the science of optics (q.v.). We here just notice its principal ' phenomena, and thnhypotlieses advanced tu explain them. Every one know s that light diverges from a luminous center in all directions, and that its transmission in any direc tion is straight. It travels with great velocity, which has been ascertained, by observa tions on the eclipses of Jupiter's satellites and other means, to be 186,000 miles per second. Shadows (q.v.) are a result of its straight transmission; and it follows from its diverging in all directions from a luminous center that its intensity diminishes inversely as the square of the distance from the center. When it falls on the surfaces of bodies, it is reflected from them regularly or irregularly, totally or partially, or is partly or wholly transmitted or refracted through them. The phenomena of the reflection and of the refraction of light are treated of' respectively- under the heads catoptrics (q.v.) and dioptrics (q.v.). The facts of observation on which catoptrics is founded are two: 1. In the reflection. of light, the incident ray, the normal to the surface, and the reflected. ray are in one plane; 2. The angle of reflection is equal to the angle of incidence. Similar to these are the physical laws on which dioptries is founded. When a ray of • homogeneous light is incident on a refracting surface, 1. The incident and refracted ray lie in the saine plane as the normal at the point of incidence, and on opposite sides of it; 2. The sine of the angle of incidence, whatever that angle may be, bears to the angle of refraction a ratio dependent only on the nature of the media between which the refraction takes place, and on the nature of the light. In stating these laws, we have hinted at light being of different kinds. At one time it was not supposed that color had anything to do with light; now, there is no serious dispute but that there are lights of different colors (see CHROMATICS and SPECTRUM), With different properties, though obeying the same general laws. Among the most striking phenomena of light are those treated of under the head Polarization (q.v.). Next to these in interest are
the phenomena of double refraction. See REFRACTION, DOUI3LE. For an account of the chief chemical properties of light, see PHOTOGRAPHY and SPECTRUM. See also for phenomena not noticed above, the articles ABERRATION, DIFFRACTION, DISPERSION, INTERFERENCE.
Two hypotheses have been advanced to explain the different phenomena of light, viz., the theory of emission, or the corpuscular theory, and the theory- of 'vibration, or the undulatory theory. According to the former, light is an attenuated imponderable substance, whose colors depend on the velocity of its transmission. It regards reflection as analagous to the rebounding of elastic bodies; while, to explain refraction, it 'assumes that there are interstices in transparent bodies, to allow of the passage of the particles of light, and that these particles are attracted by the molecules of bodies—their attraction combining with the velocity of the particles of light to cause them to deviate in their course. The undulatory theory assumes that light is propagated by the vibra tions of an imponderable matter termed ether (q.v.). On this view light is somewhat sirnilar to sound (see INTERFERENCE). Newton was the author of the former theory-, and Huygens may be regarded as the author of the latter. The theories were long rivals, but now no doubt remains that the theory of undulations has triumphed over the other. Its soundness may be said to rest on similar evidence to that which we have for the theory of gravitation: it had not only satisfactorily accounted for all the pheno mena of light, but it has been the means of discovering dew phenomena. In fact, it has suppFicd the philosopher with the power of prescience in regard to its subject. Those who wish to study the theory may advantageously consult its popular exposition by Young f lectures on Natural Philosophy, London, 1845), and Lloyd's llrave Theory of Light (Dublin, 1856). 'The mathematical theory is very fully investigated in Airy's Mathematical Tracts.