Emission Theory of Light. shot through space with immense velocity: it is now 675 910. The material or emission theory of light is so very natural that we need not wonder that it was long before men would admit any other. We smell an odour from a considerable distance, doubtless by the emanation of minute particles from the odorous body,particles far too minute to be touched or to affect any other of the senses than smell; why, then, may not light be a similar minute emission affecting only the sense of sight? The immortal Newton was the great exponent of this emission theory; and there can be no doubt that his authority weighed more in its favour than all the arguments for the rival theory did for long against it. We shall see, however, from the phenomena reviewed and described in this section, that the wave or undulatory theory of light-first prcpounded by the celebrated English philosopher Hooke in 1664, and, shortly after, greatly developed by the Dutch philosopher Huyghens-is the only one reconcilable with the multitude of otherwise inexplicable phenomena revealed by modern experimental research. Side by side with the heating beams of the sun come to us his beams of life-giving light; and side by side with the motiontheory of heat, which was explained in a foregoing section, must we accept a similar motion-theory of light. That neither heat nor light can consist of material particles, has been inferred from a variety of ingenious experiments all pointing to this conclusion. The results have been especially confirmed by an experiment made by the French philosopher Fizeau with reference to the velocity of light in liquids or media denser than air. According to the Newtonian or emission theory, the velocity should be greater in the denser medium; according to the wave theory it should be less. By a beautiful experimental device Fizeau proved that the facts of the case accord with the wave theory, and not with the other, the velocity of a beam being retarded by its passage through water. The ordinary phenomena of reflection and refraction were explained on the emission theory by Newton, with the aid of some 676 The Ether or Wave Theory of Light. subsidiary hypotheses as to the behaviour of luminous particles; but it is altogether unnecessary to recount these theories and explanations in the face of an array of optical phenomena disclosed since the days of Newton, which would require the multiplication of the characters ascribed to light by that illustrious philosopher, but which admit of complete explanation on the Huyghenian or wave theory. The Ether or Wave theory of light. 911. To account for the various phenomena of light and of radiant heat, modern philosophers assume the existence of an ether or medium of extreme tenuity and elasticity, filling all space and pervading all ordinary matter as easily as the air passes among the trees and foliage of a forest. There is some relation between this ether and grosser matter, but it is so subtle as to have hitherto eluded philosophic hypothesis. A sufficiently intense vibration of the particles of a piece of iron causes vibrations of the ether which announce themselves to our perception as heat; and a still more intense vibration of the iron transmitted to the ether reveals itself to the eye in vibrations of the retina, which we call light (Art. 554). From a variety of appearances it is inferred that the vibratory motion of the ether is not like that of the air in the case of sound, but rather like the waves seen in water, or the transversal waves artificially raised on a rope or elastic string, as already described under Acoustics (Art. 475). How a medium almost infinitely rarer than air can admit of these cross-vibrations, when the idea of cohesion, by which such an onward movement might be transmitted is out of the question, we cannot attempt to explain. But we are compelled to accept the conclusion from such facts as will be detailed in the following pages. Under this hypothesis, the reflection and refraction of light admit of ready explanation. The reflection of transversal waves follows the same law as that of longitudinal ones, or as that of liquid waves; and is at once deduced from the assumption that the velocity of light is constant for the same medium. If, on the other hand, it be assumed that within solids and liquids the density of the luminiferous ether is greater than in vacuo, owing to the interaction of gross matter and the ether, while the elasticity is also lessened by this interference of matter, it follows theoretically (and experiment justifies theory) that the light waves, on passing Analogies of Light and Sound. 677 from one medium to another, will proceed as new waves of a different velocity, with the entering surface as their origin or source. By simple geometrical reasoning, the law of refraction can be readily deduced from the latter assumption, and the remarkable conclusion arrived at that The index of refraction is simply the ratio of the veloci ties of light in the two media. 912. Thus, when a ray of light passes from air into water, the index of refraction is, as was seen in Art. 805, ; in other words, the velocity of light in air bears to the velocity in water the ratio of 3 to 2.* A further analogy between light and sound which renders the wave hypothesis of light complete, is, that the luminiferous waves are not all of one length, though they have all one common velocity. Different colours of light correspond to different wave-lengths of ether, and therefore to different numbers of vibrations of the retina per second; just as we have seen in acoustics, that different notes of the scale correspond to different wave-lengths in air and different numbers of vibrations of the tympanum per second. VR By experiments to be afterwards explained, it is calculated that the length. of an average red ray of light is about 3900th of an inch, while that of a violet one is only about 50th of an inch. Some idea of their extreme smallness may be formed when we say that the figure (237) represents a violet and a red wave-length magnified ten thousand times. These are the extreme limits of luminous waves, a very narrow range of vibrations when we compare it with the range of audible aërial vibrations. (See page 370.) The number of vibrations which strike the retina per second is inconceivably great. We may express them by figures, but the mind is powerless to grasp the corresponding reality.† The perception of colours is explained by the assumption that The absolute index of refraction is the ratio of the velocity in vacuo to that in the given substance, and is always greater than the index of refraction from air. To find the number of vibrations per second in a red ray, we divide the velocity of light per second, namely, 192,000 miles reduced to inches, by a red wave-length or inch. This gives the inconceivable number of 474 millions of millions. 678 Colours of Bodies on the Wave Theory. certain fibres of the retina are, so to speak, tuned to definite rates of vibration, and respond only to these, just as the wires of a piano vibrate in sympathy only with aërial vibrations of their own period. (See Acoustics, Art. 483.) 913. The colours of bodies, in like manner, would be explained by assuming that the surface-molecules or particles of bodies take up or quench certain vibrations of the ether, while others they refuse to absorb and send back to the eye; and it is by the latter that we form a judgment of the colour of the body. A white sheet of paper reflects all the rays of light as it receives them; a blue paper absorbs all the other colours of the spectrum (or of the rainbow) except the blue; and a red paper reflects only the red rays. In the red of the spectrum, a red wafer will have its colour intensified; in the blue, it will appear almost colourless or black, because it only receives blue rays, and these it absorbs and is unable to reflect. When light is transmitted through a coloured glass, the colouration is not due to any new property added to the beam which passes through, but is due to a quenching, sifting, or withholding of certain of the original component rays, the residue alone giving the coloured appearance to the glass. Thus, if ordinary white light be passed first through a red glass, all the blue and green rays are quenched or sifted out of the original beam; and if the red ray be viewed through a blue glass it will appear almost black. Blue glass transmits only blue rays, not red; hence if only red fall on the blue glass no light at all will pass through. The undulatory theory of light, in accordance with which the phenomena of the spectrum have been already explained and satisfactorily accounted for, receives further confirmation from another and entirely different class of phenomena, which will be now briefly described. 914. Interference.—The phenomena to which we refer are those due to the interference of luminous waves of the same intensity, whereby two lights may increase each other's effects, or may partially or entirely destroy one another, and so produce darkness out of light.* There are various natural and experimental methods *These facts, as Mr. Tomlinson remarks, are totally unintelligible if light is regarded as matter; for two material particles cannot annihilate each other, as the rays of light do, and as two forces or motions can. Interference of Waves. 679 whereby this interference may be produced, but the fundamental principle is the same in all, and may be easily understood. Suppose that, as shown in fig. 238, we have two chains, A C, BC, united to one common chain, CD, and suppose that, as explained in the section on Acoustics, p. 309, we have the means of sending cross or chain-waves along each towards CD (as represented by the dotted lines in the figure), then it is obvious that the disturbance of the end, C, of the chain, C D, will depend on the relative phases of the waves approaching from the two different quarters. That Fig. 238. is to say, if a crest of a wave from A coincide with a hollow of a similar wave from B, then C will remain undisturbed, the one motion just extinguishing the other. If, on the other hand, the different waves coincided in direction when they arrived at C, then a wave of double amplitude would pass along the chain, C D; or, to put it in general terms, the wave-motion set up at C will be the compound or resultant of the individual wave-motions arriving from A and B. 915. An exactly analogous case of interference of sonorous waves may be easily exhibited experimentally. If, with a violin-bow, we sound the fundamental note of a square metal plate fixed at the centre, we know (see Art. 542) that the adjacent quarters of the plate are vibrating in opposite phases, the nodal lines forming a central cross, as in the figure. On holding a Y-shaped tube, closed with an elastic membrane, M, over the plate, we find that sand strewn on the membrane will remain quiescent when the Y-tube is placed as in fig. 239, because the aërial pulses sent by B and C are in exactly opposite phase, and just counteract each other's effects. Whereas, if we hold the Y-tube diagonally over A and C, the sand is violently agitated. A still simpler acoustic experiment, illustrative of interference of waves, is that of striking a tuning-fork sharply, holding it about two feet from the ear, and turning it slowly round. A position is easily found where the sound of the fork is inaudible, the pulsations from the one leg just counteracting those of the other leg. Fig. 239. |