Wave theory of light

“ And as the inclination is increased the part of the light transmitted will diminish, until, when the second Nicol is in a position to transmit only vertical vibrations (i. e., when it has turned through a right angle) , the light will vanish. Such is an explanation of this fundamental experiment in polarization on the principle of what is called the Wave Theory of Light ”

“ The emission theory died hard and the early advocates of the undulatory theory of light were forced to work up with a completeness, probably without parallel in the history of science, the evidence, necessarily indirect, that in optics we have to do with a wave-motion. The standpoint of optical theory may be deemed conclusive, possibly final, so far as the general proposition is concerned that it is the science of a wave-motion. ”

“ The answer of the wave theory to the objection now is, that light does bend round corners though only slightly and that the smallness of the bend is quite simply due to the extreme shortness of the light waves. The longer waves are, the more they bend round corners. ”

“ It would seem that what the light-waves were really bearing within reach of each atom was not a millionth of a quantum but a millionth chance of securing a whole quantum. The wave-theory of light pictures and describes something evenly distributed over the whole wave-front which has usually been identified with energy. Owing to well-established phenomena such as interference and diffraction it seems impossible to deny this uniformity, but we must give it another interpretation; it is a uniform chance of energy. ”

“ The transmission of a disturbance or vibratory motion by means of waves, as in the case of sound, was a well-recognized principle, and Young and Fresnel applied it most successfully to the phenomena of light. Wave motion, in a general way, is only possible in a sensibly continuous medium, such as water, air, etc., and the theory that light was a vibratory disturbance transmitted by means of waves necessitated the assumption of the existence of such a medium throughout all space in which light travelled. ”

“ The wave theory of sound has long been impregnable; but these beautiful analogies between light and sound, though provided for by theory, have been experimentally demonstrated only recently. Such new and unexpected confirmations, new points of contact, are always welcome, even though they be not needed for the establishment of a theory. They are the results of prevision based on the assumption that an elastic material medium is needed for the propagation of sound, and are wholly inexplicable on any theory of emanation analogous to Newton's emission theory of light. ”

“ Huyghens, on the contrary, supposes light to be caused by the waves or vibrations of an infinitely elastic medium—Ether—diffused through all space, which waves are propagated in every direction from the luminous body. In the first theory, a luminous particle is supposed actually to come from the sun to the earth; in the other, the sun only occasions a disturbance of the ether, which extends with great rapidity, in the same manner as a wave spreads itself over the surface of a lake. ”

“ It is easy to see that this retardation of the one end of the wave must cause it to swing round and change its front, so that when the wave has fully entered the glass its course is oblique to its original direction. According to the undulatory theory, light is thus refracted.
With these considerations to guide us, let us follow the course of a beam of monochromatic light through our glass prism. The velocity in air is to its velocity in [Pg 108] glass as 3: 2. Let A B C (fig. 25) be the section of our prism, and a b the section of a plane wave approaching it in the direction of the arrow. ”

“ DOES THE SPEED OF LIGHT IN SPACE DEPEND UPON ITS WAVE-LENGTH?
When a beam of light comes through a prism of glass or a raindrop it is dispersed into a band of vivid colors, each denoting a particular wave-length. Though all these wave-lengths travel together in the air they part company in the glass or the water because there they no longer possess the same speed. The long waves, which produce the sensation of red, travel faster than the short, or violet waves. ”

“ Accordingly, the experiment of producing interference between two sets of light rays so that they destroy each other is a strong argument in favour of the view that light must consist in some kind of wave-motion existing in a medium susceptible of supporting it, filling all space, and existing in all transparent bodies. ”

“ At the extreme violet end of the spectrum, for example, the wave-length is about fifteen millionths of an inch, at the red end it is about twice as great; from which it follows (§ 283) , from the known velocity, of light, that when we look at the red end of a spectrum about 400 billion waves of light enter the eye per second, and twice that number when we look at the other end. Newton's experiment thus shews that a prism sorts out light of a composite nature according to the wavelength of the different kinds of light present. ”

“ The waves generated in the ether by the swinging atoms of luminous bodies are of different lengths and amplitudes. The amplitude is the width of swing of the individual particles of the wave. In water-waves it is the height of the crest above the trough, while the length of the wave is the distance between two consecutive crests. The aggregate of waves emitted by the sun may be broadly divided into two classes: the one class competent, the other incompetent, to excite vision. But the light-producing waves differ markedly among themselves in size, form, and force. ”

“ When light is scattered by particles which are very small compared with any of the wave-lengths, the ratio of the amplitudes of the vibrations of the scattered and incident light varies inversely as the square of the wave-length, and the intensity of the lights themselves as the inverse fourth power. ”

“ If light is a substance, no possible explanation can be given which will enable us to account for the combination of two rays of light producing darkness at their meeting-point. If, on the other hand, rays of light consist of waves of some kind in a medium, then, as we have seen in the case of water ripples and air waves, it is quite possible for two wave-trains to annihilate each other’s effect at a certain point, if a hollow of one wave-train reaches [191] that place coincidently with a hump belonging to the other. ”

“ The light that comes from the sun is made up of waves of various lengths; but making it pass through a prism we can separate it out into a spectrum, and in that way we find a band of light instead of a spot coming from the sun, and to every band in the spectrum corresponds a wave of a certain definite length and definite time in vibration. Now we come to a very singular phenomenon. If you take a gas such as chlorine and interpose it in the wave of that light, you will find that certain particular rays of the spectrum are absorbed, while others are not. ”

“ When light is analysed by a prism or grating into a spectrum, the course of each ray is deflected—refracted or diffracted—by an amount corresponding to its frequency of vibration or wave-length.
Motion of the medium, so long as it is steady, affects neither frequency nor wave-length, and accordingly is without influence on the result. ”

“ The physical effect we call light, and that which we have up to the present moment merely called electric radiation, are really identical in nature, and both consist in waves propagated through the space-filling æther, the only difference between them is in wave-length and wave-amplitude. ”

“ In the same way if the rate of vibration of a wave of light is in accord with that of a molecule or atom, the amplitude or swing of the atom or molecule is increased, and the energy of the wave and therefore its amplitude is totally or partially destroyed; and as the amplitude is a function of the intensity of the light, the ray fails to be seen at all, or else is diminished in brightness. ”

“ For the force of these waves must grow feeble in proportion as they move away from their origin, so that the action of each one in particular will without doubt become incapable of making itself felt to our sight. But one will cease to be astonished by considering how at a great distance from the luminous body an infinitude of waves, though they have issued from different points of this body, unite together in such a way that they sensibly compose one single wave only, which, consequently, ought to have enough force to make itself felt. ”

“ Light propagates through space as a wave. The amplitudes, wavelengths, frequencies, and speed of light are affected by how light is emitted and the particular conditions of the material through which it is propagating. ScienceApologist (talk) 20:23, 20 April 2009 (UTC) The frequency of light waves is determined by the period of the oscillations of the emitter. While the frequency of a light wave does not normally change as light travels through different materials, called optical media, the wavelength of light is affected by the indexes of refraction in different media. ”

“ It was this fact that Professor Hertz discovered, and the waves are sometimes called the Hertzian waves. Light is one variety of wave motion, and heat another. The ether must be distinguished from the air, for science means by it a medium which exists everywhere and is to be regarded as permeating all space and all matter. The ether exists in a vacuum, for, although all the air may have been withdrawn, an object placed in a vacuum can still be seen from outside, and hence the wave motions of light are traveling through a space devoid of air. ”

“ But such longitudinal waves have no place in any optical phenomenon, and therefore constitute a difficulty for the theory. In order not to have them it was necessary for Green to suppose the ether incompressible. Thus the theory did very well for the propagation of light on free space. When light passes from free space to a transparent substance, however, it is partially reflected and partially refracted, travelling with a different velocity in the new medium. This change of velocity could be explained by a difference of either density or rigidity in the two media. ”

“ Light is therefore an influence propagated as wave-motion, and moreover by transverse undulations, for the reasons brought out by Thomas Young and Augustin Fresnel; so that the aether is a medium which possesses elasticity of a type analogous to rigidity. It must be very different from ordinary matter as we know it, for waves travelling in matter constitute sound, which is propagated hundreds of thousands of times slower than light. ”

“ So for a hundred years or more people have, generally speaking, accepted the idea that light consists of waves in a medium called The Ether. Heat also is brought to us from the sun and from any distant hot body by similar means, the difference between light waves and heat waves being simply in their wave length or the distance apart. The different colours of light, too, are to be accounted for by different wave lengths. ”

“ Light, the stimulus, is physically a wave motion, its vibrations succeeding each other at the rate of 500,000000,000000 vibrations, more or less, per second, and moving through space with a speed of 186,000 miles per second. The "wave-length", or distance from the crest of one wave to the crest of the next following, is measured in millionths of a millimeter.
The most important single step ever taken towards a knowledge of the physics of light, and incidentally towards a knowledge of visual sensations, was Newton's analysis of white light into the spectrum. ”

“ From the fact that all Hertz waves travel with the same speed as light, and from our ability to imitate, as you have seen, the well-known optical phenomena with Hertz radiation of short wave-length, the great induction has been made that all æther waves have the same essential nature, and that invisible actinic rays, light rays, dark-heat rays, and Hertz rays are all of them æther waves of various wave-lengths and amplitudes. ”

“ But we can extend our investigations in the other direction; we find that the ether carries many waves longer than light-waves. Special photographic emulsions can reveal the existence of waves five times longer than violet-light waves. Extending below the limits of visibility are waves we detect as heat-waves. Radiant heat, like the heat from a fire, is also a form of wave-motion in the ether, but the waves our senses recognise as heat are longer than light-waves. There are longer waves still, but our senses do not recognise them. ”

“ The result is in general that in the final stages the surface will be marked by a series of groups of waves of diminishing amplitude separated by bands of comparatively smooth water. The fact that the wave-velocity of a simple-harmonic train varies with the wave-length has an analogy in optics, in the propagation of light in a dispersive medium. In both cases we have a contrast with the simpler phenomena of waves on a tense string or of light waves m vacuo, and the notion of " group-velocity, " as distinguished from wave-velocity, comes to be important. ”

“ Optics has nevertheless certain distinctly speculative features. Wave-motion demands a medium. The enormous velocity of light excludes known forms of matter; the transmission of radiation in vacuo and through outer space from the most remote regions of the universe, and at the same time through solids such as glass demands that this medium shall have properties very different from that of any substance with which chemistry has made us acquainted. ”