Wavelength

“ Natural light is the result of waves oscillating at a tremendous speed per second, just as other waves, heavier and travelling far more slowly in comparison with light waves, produce sound. Our familiar conception of natural light, or daylight, is that it is white; but we also know that when a narrow beam is permitted to fall upon a length of glass of triangular section, known as a prism, the light which on the outer side appears white is found to be resolved into colours on the opposite side. ”

“ Now, as light is supposed to be produced by waves or undulations of an ethereal medium filling all nature, and occupying the pores of the transparent bodies; and as sound is produced by undulations or waves in the air: so the successive production of light and darkness by two bright lights, and the production of sound and silence by two loud sounds, may be explained in the very same manner as we have explained the increase and obliteration of waves formed on the surface of water. ”

“ That which before lay buried in the shade of night is itself now a radiator of the luminous fluid. Mark the genial warmth that comes as the sister of light; then stand by the side of the experimentalist and watch the point on which he directs the shining focus, and in an instant see iron melt and stones run like water, under the fervent heat! Now look upward to the heavens, where the falling drops of rain have formed a natural prism in the rainbow, and shown that the beam of pure whiteness, refracted into various rays, glows with all the tints that adorn the garden of nature. ”

“ Thus, then, in the case of sound, we have the sonorous body, the air, and the auditory nerve, concerned in the phenomenon; in the case of light, we have the luminous body, the ether, and the optic nerve. The fundamental analogy of sound and light is thus before us, and it is easily remembered. But we must push the analogy further. We know that the white light which comes to us from the sun is made up of an infinite number of coloured rays. By refraction with a prism we can separate those rays from each other, and arrange them in the series of colours which constitute the solar spectrum. ”

“ We see light, the waves of which, or ether vibrations, are transversal; air waves or sound vibrations are longitudinal.
We have spoken of radiant heat. Light acts in the same way through the ether; and when we consider Sound we shall learn that a certain number of vibrations of a string give a certain sound, and the quicker the vibration the shriller the tone. So in light. The more quickly the waves of luminosity travel to our eye, and the faster they strike it, the greater the difference in the colour, or what we call colour. ”

“ Light is thrown out on all sides from a luminous body like the spokes of a cart-wheel, and in the absence of any obstruction, the rays are distributed equally on all sides, diverging like the radii drawn from the centre of a circle. As a natural consequence arising from the divergence of each ray from the other, the intensity of light decreases as the distance from the luminous source increases, and vice versâ. ”

“ By causing the light to pass through a prism the mixed rays are sorted out side by side according to their several wave-lengths, forming a [Pg 12] broad, many-hued band or “spectrum” upon a white screen placed to receive it. (See Fig. 1.) To the visible rays of the longest wave-length is due the red colour on the extreme left. Waves of somewhat shorter length produce the adjoining stripe of orange, and the succeeding colours—yellow, green, and blue—correspond respectively to waves of shorter and shorter lengths. ”

“ The whole body of vibrations caused by the sun is termed radiation, or, more commonly, rays; and consists of sets of waves which vary considerably in their dimensions and rate of recurrence, but of which the middle portion only is capable of exciting in us sensations of light and colour. Beginning with the largest waves, which recur at the longest intervals, we have first those which produce heat-sensations only; as they get smaller and recur quicker, we perceive a dull red colour ”

“ According to the undulatory hypothesis, light is not a mass of molecules emitted by a luminous body, but simply the vibration of an elastic fluid which is conceived to fill the whole of space. A comparative example may assist you in understanding this theory more clearly. If you throw a stone into a smooth piece of water, there will form around the point where the stone fell, a series of circular undulations, starting from the centre and gradually enlarging themselves. If a loud noise is suddenly heard, the same effect is produced round the point from whence the sound proceeds. ”

“ We may note, in passing, that it is not every transparent body when fashioned into a prism which thus analyzes the light into a fan-shaped beam with rays of various wave-lengths arranged in the order of their wave-lengths. ”

“ The most important fixed lines in the spectrum formed by light emitted from the sun, whether it is reflected from the sky, the clouds, or the moon, may be easily seen by looking at a narrow slit in the window-shutter of a dark room, through a hollow prism formed of plates of parallel glass, and filled with any fluid of a considerable dispersive power. ”

“ Emily. But do not the rays which are projected in different directions, and cross each other, interfere, and impede each other's course?
Mrs. B. Not at all. The particles of light, are so extremely minute, that they are never known to interfere with each other. A ray of light, is a single line of light, projected from a luminous body; and a pencil of rays, is a collection of rays, proceeding from any one point of a luminous body, as fig. 2.
Caroline. Is light then a substance composed of particles, like other bodies? ”

“ I. When objects are lighted by the direct rays of the sun, or by direct light entering from a window, one side of them is of course in light, the other in shade, and the forms in the mass are exhibited systematically by the force of the rays falling on it; (those having most power of illumination which strike most vertically;) and note that there is, therefore, to every solid curvature of surface, a necessarily proportioned gradation of light, the gradation on a parabolic solid being different from the gradation on an elliptical or spherical one. ”

“ If light of one wave length is used, as red light, the regions of reinforcement and interference are shown by red and dark rings, while if white light is used, the ring where red light interferes, yields its complementary color, greenish blue. ”

“ Two bright lights may be made to produce darkness.—If two pencils of light radiate from two spots very close to each other in such a manner that they cross each other at a given point, any object placed at that line of interference will be illuminated with the sum of the two luminous pencils. If we suppose those rays to move in waves, and the elevation of the wave to represent the maximum of luminous effect, then the two waves meeting, when they are both at the height of their undulation, will necessarily produce a spot of greater intensity. ”

“ A wave of a certain velocity and length gives us a certain sensation which we call blue; another awakens the sensation we call yellow. The two series of waves, mingling, produce a new sensation which we call green. The necessity of reflection for the production of these sensations is evident. The mingled waves have no color in their incident flow; but, striking some object, these waves become separated, some being absorbed, and the reflected ones produce the peculiar sensation we call color.
We know that these varying conditions of light which affect us as color have an absolute being. ”

“ We observe, for instance, in his chapter on Sensation (p. 236 seq.) that all light-waves are the same in kind of movement, but differ in size as widely as the ripples on a sea-beach (round which happy children play) differ from the vast Atlantic ship-engulfing billows sixty or a hundred feet apart. All these undulations are similar in respect of reflection, refraction, interference, diffraction, and polarisation, as well as in their production of heat. [124] Now, it is the interpretation of such movements into its own language by which our eye gives us the sensation of colour. ”

“ In passing through the prism the beam of light has been deflected from a straight path, and the result is that the waves of varying length, of which white light is composed, have been sorted out. The shortest rays are red, while the longest rays have a violet tinge. The analysis of light, as revealed by the spectrum in a dark chamber, is performed for us by Nature in the rainbow. ”

“ The reflected light waves not only make us see things, but they make us see them in different colors. Imagine a sunbeam playing on a leaf: part of its waves bound straight back from it to our eye, and make us see the surface of the leaf ”

“ This is a narrow limit, and on either side there must be rays, which eyes or nerves differently constructed would receive and interpret, yielding, perhaps, colors unknown to our consciousness. There is a harmony in color waves, like music in sound waves, for as a note blends in one, in all octaves above or below, so light waves, twice or thrice the length of given waves yield the same color impression. ”

“ The phenomena of the interference of beams of light in certain circumstances, to produce darkness or colour, indicate that light is some form of wave motion, and, to carry these waves, a hypothetical luminiferous aether was invented. ”

“ 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. ”

“ At the outset we have such purely physical questions as the nature of light, the cause of its emission, the mode of its propagation, the difference in the waves which give rise to the various color sensations, the principles of absorption, of reflection and of refraction, and the nature of material surfaces whereby they acquire their characteristic colors. ”

“ But light of one pure colour may be compared to sound of one determinate note. As the aerial waves producing the effect of one definite tone are all of one length, so the ethereal waves producing light of one definite colour are all of one length. Therefore if we approach or recede from a source of light emitting such waves, effects will result corresponding with what has been described above for the case of water-waves and sound-waves. If we approach the source of light, or if it approaches us, the waves will be shortened ”

“ In one word, the sphere of darkness in which I floated seemed to be filled with points of light, while the absolute blackness that surrounded them, the absence of the slightest radiation, or illumination of space at large, was strange beyond expression to an eye accustomed to that diffusion of light which is produced by the atmosphere. ”

“ The light of this orb is subject to three principal laws requiring notice, viz., transmission, colour, and refraction by transparent media.
1st. The sun or any body in a state of combustion contains independent light; and striking opaque or non-luminous bodies in straight lines or rays, which are intercepted and reflected at the same angle as that of incidence. When these same rays strike the eye of man, the illuminated body becomes visible. ”

“ 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. ”

“ Thus white light is a compound thing containing long waves with which heat phenomena are associated, waves of medium length to which alone the eye is tuned, and short waves which have a decided action on some metallic salts which are unaffected by the others. ”

“ A transparent Body which looks of any Colour by transmitted Light, may also look of the same Colour by reflected Light, the Light of that Colour being reflected by the farther Surface of the Body, or by the Air beyond it. And then the reflected Colour will be diminished, and perhaps cease, by making the Body very thick, and pitching it on the backside to diminish the Reflexion of its farther Surface, so that the Light reflected from the tinging Particles may predominate. ”

“ Longer wave lengths are towards the red end of the spectrum, shorter ones towards the violet end. When the source of light is moving towards you, the apparent wave [Pg 135] lengths grow shorter, just as waves at sea come quicker when you are traveling against the wind. ”

“ According to Tyndall, when very minute solid particles, smaller than the luminous waves, are diffused in a medium traversed by light, the light is decomposed in such a way that the least waves, the blue ones, predominate in the reflected rays, and the largest ones, the red waves, in the transmitted rays. ”

“ Light is transmitted by waves, similar in some ways to waves in water, the light waves being disturbances of the light-bearing ether, an invisible, imponderable substance of zero density and infinite elasticity, which pervades all matter. ”

“ Light naturally travels in a straight line. It is deflected only when it passes from one transparent medium into another—for example, from air to water—and the mediums are of different densities. We may regard the surface of a visible object as made up of countless points, from each of which a diverging pencil of rays is sent off through the ether. ”