Light. The general doctrine of Light is now only a part of the general theory of Radiation, which comprises Radiant Heat and Actinic and Electric Radiation as well as Light; but, since the battle as to the nature of radiation in general was first waged round light itself, it is convenient to consider light as representing all the forms of radiation. By elementary observation it is found that light travels (within a uniform medium) in straight lines in all directions; that it accordingly, when proceeding from a point or a very small source, covers areas which vary as the squares of the distance; that shadows obey a similar law; that it is seen some time before the corresponding sound is heard: all which points to a quick propagation of something in straight lines. What is this which is propagated—matter, motion, or condition? The simplest explanation was that luminous bodies emitted something material whose impact affected the sensitive eye; that the reflection of light at surfaces was due to elastic rebound of this quick-travelling material. These phenomena might be equally well explained by waves travelling and being reflected; but Sir Isaac Newton could not reconcile himself to the notion of waves travelling in straight lines and not spreading. This difficulty, which we now know not to be a real one, inasmuch as it is only a question of proportion between the breadth of the wave-front and the distance between successive waves whether a wave-motion shall or shall not travel in straight lines, led him to adopt and develop the corpuscular or emission theory of light. According to this all luminous bodies emit with equal velocities (a troublesome postulate, since the retarding attraction of the sun is so much greater than that of a candle-flame) a number of elastic corpuscles (whose mass must be extremely small, otherwise, with the velocity of more than 186,000 miles per second, their momentum would be destructive), which travel in straight lines, are reflected, and are refracted (provided that they travel more rapidly in the denser medium than in air or in vacuo, in a direction at right angles to the bounding surface between the rarer and the denser medium). But here begin the difficulties: refraction is always accompanied by reflection, whence some corpuscles enter the denser medium, some rebound; hence a theory of easy fits of reflection and transmission had to be developed, and this involved as its explanation a theory of vibration of a general medium some way in advance of the travelling corpuscles, so as to aid or check their entrance into the denser medium. Newton discovered that the different colours of the spectrum were unequally refracted in glass; from this he had to infer that there were as many different kinds of molecules emitted as there were colours in the spectrum. Then, again, shadows are not absolute; the shadow of a hair produced by sunlight passing through a minute pinhole in which stands a droplet of water is bright in the centre; hence explanations had to be provided to account for the bending of rays round an object: then these explanations failed to account for similar phenomena observed when light was reflected from two mirrors. The theory became loaded with a mass of hypotheses devised to explain each particular phenomenon; but the great authority of Newton maintained its vitality down to the time and person of Sir David Brewster.
The wave-theory of light was suggested by Grimaldi, Hooke, and others; was formed by Huygens (1678), who explained double refraction; lay in abeyance until Young revived it at the beginning of the 19th century; was developed by Fresnel (1815 onwards); and has now definitely displaced the emission-theory. According to it light consists of vibrations in an all-pervading elastic jelly-like ether; the vibrations are, unlike those of sound, in directions at right angles to the direc- tions of propagation; and 'rays of light' are mere lines showing the direction of propagation of (and in isotropic media at right angles to) the corresponding portions of the wave-front. This theory involves the admission of a vibrating ether; so, indeed, did Newton's. The transmission in straight lines is easily explained; points lying to one side are not affected, because different parts of the wave-front neutralise one another's effects, if the wave-length be relatively small: even sound travelling through large apertures travels in straight lines. The wave-theory readily explains reflection; in refraction it assumes that the waves travel less rapidly in the denser medium (which is found to be true, and therefore disposes crucially of the emission-theory), and it explains the accompanying reflected wave; it explains double refraction, polarisation, interference, colour (different wave-lengths), diffraction, &c. Dispersion is not yet completely explained, because data are wanting, though Cauchy's ideas, as developed by Sir William Thomson, have shown that this is a phenomenon of waves of different lengths passing through finely non-homogeneous matter. The wave-theory has also proved the means of forecasting most recondite and unexpected phenomena.
But, then, what is a 'wave?' It is not necessarily a wave of motion. All the phenomena are explicable as phenomena of rhythmical disturbance of some kind, and the 'wave-theory' really goes no further than to state this. The rhythmical disturbance may be one of position (wave-motion), of stress, of electrical condition, possibly of twist in the ether. According to Clerk-Maxwell's theory every portion of the ether in the path of a beam of light is subject to rapidly-alternating stresses transverse to the ray, and is therefore in a rapidly-alternating electric and also in a rapidly-alternating magnetic condition; and the curious relations now known to exist between beams of light and the field of force of a magnet lend the greatest probability to this theory. Besides, Hertz's discoveries (see MAGNETISM) have shown that phenomena of exactly the same character as those of light, and differing only in wave-length, exist in the magnetic field while induction is going on; and all the phenomena of light, radiant heat, and actinic radiation are reduced to phenomena of electromagnetic radiation between certain limits of wave-length.
The velocity of light is found by timing the eclipses of Jupiter's satellites when they are at the greatest and the least distance from the earth, by astronomical aberration-observations, by finding (Fizeau) what speed must be given to a cog-wheel to make it rotate one tooth's-breadth while light is going to a given distant mirror and returning, by finding (Foucault) what position is ultimately assumed by a ray which travels from a source to a rotating mirror, thence to a distant mirror, and thence back to the original mirror, which by this time has been rotated somewhat. In the last method it is found that the interposition of optically denser or more refractive media retards the light. The mean of all observations is that light of all wave-lengths travels in vacuo with a velocity of 30,057,400,000 centimetres or 186,772 miles per second; in air with a velocity less than this in the ratio of 10,000 to 10,003. The length of waves can be ascertained from measurement, at a sufficient distance, of the fringes produced by Interference (q.v.), or by the use of diffraction-gratings ruled with lines to the centimetre, in which case the wave-length for any particular colour is in centimetres the th part of the sine of the angle of deflection of that colour in the first 'diffraction-spectrum,' a result easily reached through the general theory of waves. The wave-lengths of radiant heat, light, and actinic radiations range from cm. or inch (the longest invisible heat-rays, Langley) to cm. or inch (invisible actinic rays); the visible limits are and cm. The frequencies or number of waves per second accordingly range from 20 millions of millions to 40,000 millions of millions per second, the extreme visible limits being 392 and 757 millions of millions per second.
See ABERRATION, DIFFRACTION, DISPERSION, ETHER, INTERFERENCE, OPTICS, PHOTOGRAPHY, POLARISATION, RADIATION, REFLECTION, REFRACTION, SHADOWS, SPECTRUM; and Tait's Light, Glazebrook's Physical Optics, Stokes's Burnett Lectures on Light.