Page:The New International Encyclopædia 1st ed. v. 12.djvu/265

LIGHT. 239 LIGHT. uiitted to the Roj-al Society. Gi-egory had con- structed an instrument on similar principles some years befoi'e. About the same time, Gri- maldi made his interesting series of experiments on diffraction, and noticed the remarkable fact of the interference of one pencil of liglit with the action of another. To explain the phenomenon of sharp shadows Xewton advanced his "corpuscular' theory of light, the idea of which was that light is due to the emission of streams of fine particles from the source of liglit. To explain refraction on this theorj' it was necessary to assume that the velocity of light is less in air than in glass or water. Using this theory, Xewton tried to ex- plain diffraction and the colors of thin plates; but the hj-potheses involved in the explanation were too involved to be safisfaclory. The im- portant services of the ingenious but eccentric Hooke cannot be easily stated in a few words, as he discovered a little of everything, completed n<ithing. ami occupied liimself to a large extent in combating faulty points in the theories of his contemporaries. It must not, however, be for- gotten that he has as much right as Huygens In the credit of originating the undulatory theory, though Ilooke made little more than a lucky guess, while Iluygens gave a remarkalde discussion of the application of the theory to reflection and refraction. Xewton's corpuscular theory was, however, the accepted one until the work of Young and Fresnel. The double refrac- tion of Iceland spar was discovered (1060) by Bartholin, and fully explained in 1690 by Huygens. The velocity of light was discovered by Roemer (1075), and in 1720 the aberration of the fixed stars and its cause were made known by Bradley, who likewise determined with ac- curacy the amount of atmospheric refraction. The fact that the two rays produced by Iceland spar were 'polarized' was known to Huj-gens; luit polarization by rctlection was not known until discovered by JIalus in 1808. The proper- ties of polarization were then investigated by Brewster, Biot, and especially by Arago. The proof that light is due to wave-motion was first given by Dr. Thomas Young, "who pub- lished his work on interference in 1801. The diinculty of accounting for polarization phe- nomena by the theory of waves was first met by Fresnel, who proposed the idea that the ether- waves are transverse, and showed how this hypothesis perfectly explained the observations. It is to Fresnel also that we owe the explanation of rectilinear propagation and of diffraction. The attempt made by Fresnel to give a dynamical theory of light was not successful, although he did deduce formul;i> for reflection, refraction, and total reflection which are in good accord with experiment. Green, Lord Kelvin. Helmholtz, Stokes, Eayleigh, and more recently Poincare, T.armor, and Lorentz, have advanced dynamical tlicories with more or less success ; the theory of Lorentz based on the motion of electrons is most satisfactory. GEOMETRICAL OPTICS. It is assumed in this subject — as the result cither of direct experiments or of deduction from physical optics — that 'rays of light' pass in straight lines through any homogeneous medium; that they are independent of each other: that when a ray meets a surface separating two trans- parent material media, e.g. water and air. it produces a reflected ray back into the first medium and a refracted ray in the second medium, the two having definite directions given by the laws of reflection and refraction. The laws of reflection are: If a tangent plane is drawn to the separating surface at the point where the ray strikes, the line perpendicular to this plane at the point bisects the angle between the inci<lent and reflected rays and lies in the plane which includes them. The laws of ordinary refraction are: If a tangent plane is drawn to the separating surface at the point where the ray strikes and if a perpendicular line to this plane is drawn through this point, it will lie in the plane including the incident and refracted rays; and if the angles between these rays and the perpendicular line are 9, and 0^, the ratio of sin Bi to sin 6, is a constant for the two media for a given kind of light. This ratio is called the "index of refraction of the second medium with reference to the first;' it is found to vary for light of different colors. If the first medium is the pure ether, this ratio is called the index of refraction of the second medium. There are certain bodies — either crystal or isotropic bodies in a state of strain — such that when an incident ray falls ujion them, not one, but in general two refracted rays are produced. This is called 'double refraction.' In one class of bodies one of the refracted rays obeys the ordinary laws of refraction and is called the "ordinary ray,' while the other ray does not in general do so and is called the 'extraordinary' ray. In other bodies neither of the refracted rays obeys the ordinary laws in general. The main features of bodies of the fm-mer class, e.g. Iceland spar, were fully ex])lained by Iluygens on the wave theory, while those of the latter class, e.g. aragonite, were explained similarly by Fresnel. See section on Physical Optics below. Sh.dows. If rays pass in straight lines, the main phenomena of shadows are at once ex- plained. A small source of light, a 'point- source,' will cast sharp shadows of any opaque object which stops the rays. If the source of light is large, like a window, there will be cer- tain points in the shadow of an opaque object, close to the object, which are not reached by any rays, there will also be points which are reached by rays from pai't of the soirce only, and then there will be points which receive rays from the whole source. Those points which do not receive any rays lie in the 'umbra' or shadow projier, ^^•hile those which receive rays from only a portion of the source lie in the 'penumbra.' Thus, an eclipse of the sun by the moon is totnl at a point of the earth's surface which passes through the shadow cast out into space by the moon obscuring the sun's rays; it is called par- tifd at a point on the earth if that point passes through the penumbra only; it is called 'annular' if at any time during the eclipse there can be seen a ring of the sun's surface extending past the moon's disk. 'Pin-hole photography' is an- other phenomenon which follows at once from the passage of rays in straight lines. IvKFi.ECTiox. If rays from a point-source fall upon a reflecting surface, each ray individually will obey the laws of reflection. If the incident rays form a small solid cone symmetrical about the line drawn through the point-source and ])erpendicular to the surface, they are said to