680

Waves of Light and Sound.

Liquid waves offer another illustration. If two waves of water arrive from different sources at the same spot at one and the same instant in such a manner that the crest of the one coincides with the hollow of the other, they will just destroy each other; whereas, if they coincide crest with crest, they will intensify each other's effects. “A magnificent example of these effects is seen in the well-known phenomena of the spring and neap tides; the tidal wave in the former case being the sum of the waves caused by the action of the sun and moon; and in the latter, their difference. The peculiarity of the tides in the port of Batsha furnishes a still more striking instance of the principle of interference. The tidal wave reaches this port by two distinct channels, which are so unequal in length that the time of arrival by one passage is exactly six hours longer than by the other. It follows from this, that when the crest of the tidal wave, or the high water, reaches the port by one channel, it is met by the low water coming through the other; and when these opposite effects are also equal, they completely neutralise each other. At particular seasons, therefore, when the morning and evening tides are equal, there is no tide whatever in the port of Batsha; while at other seasons there is but one tide in the day, whose height is the difference of the heights of the ordinary morning and evening tides.” *

As a mere inference, then, of the undulatory theory, the analogy of two sounds producing silence would warrant us in arriving at the remarkable conclusion that two lights may produce darkness. But the most convincing experimental evidence has been brought to bear on this conclusion. We owe to Dr. Thomas Young the discovery of this great principle of interference, and the beautifully simple explanation which it gives of phenomena inexplicable on the old material theory of light.

916. The following experiment, due to Grimaldi, became more decisive in the hands of Dr. Young. If a beam of sun-light be admitted through two small similar holes in the shutter of a darkened room, the diverging cones of light ultimately meet and overlap; and if the light admitted be simple, that is, all of one colour or wave-length, then it is found by catching the over-lapping images on a screen, that there are a series of alternate bright and black bands. These bands disappear if one of the beams be cut off, and the dark intervals recover their brightness, proving conclusively that the darkness

* Lloyd's 'Wave Theory of Light.'

Measurement of Waves of Light.

681 was due to the collision of the one set of luminous rays with the other.

B

D

E

G

Fig. 240.

Fig. 240 will show how the length of the ethereal waves may be calculated from this experiment. A and B represent the two openings, or sources of light, of similar wave-length; the lines drawn from each represent the diverging rays. First of all, at a point, E, exactly between A and B, two similar waves which had started together from the same source, the sun, will arrive at the same instant, and therefore in the same phase; hence their effects will conspire, and E will be a bright spot. If, again, D he a point, such that A D is shorter than B D by half a wave-length; then two waves, starting together from A and B will be in opposite phases when they meet at D; and the crest of the one will just annul the hollow of the other, or D will be a dark band: lastly, if C be situated so that A C be just one wave length less than B C, then waves leaving A and B simultaneously will coincide, crest with crest, at C, and their effects will in consequence conspire, or C will indicate a bright band. It is easy from this to see that, knowing the distance of the screen from A and B, and measuring accurately the intervals between the dark bands, we can estimate the wavelengths corresponding to the colour of light transmitted through the apertures, A and B. If red rays are first transmitted, as by placing a glass of that colour in front of the minute openings, A and B, and then blue rays transmitted, it is found that the bands are nearer for blue rays than for red, a fact which agrees exactly with the foregoing statements as to wave-length of the colours of the spectrum (Art. 912).

917. In this way, by accurate measurement of the distance between the openings, the distance of the screen, and the breadth of the lines, it has been estimated that the length of a wave at the extreme end of the spectrum is 266 ten millionths of an inch; and that of a wave at the extreme violet end is 167 ten millionths; or that the average length of a wave of light is 203 ten millionths of an inch; that is to say, there would be about 50,000 of them in the space of an inch. Hence heat rays are longer than chemical rays, as the former belong especially to the extreme and ultra-red part of the spectrum, while the latter belong chiefly to the violet end (Art. 908).

682

Coloured Spectra from Interference.

The famous French philosopher Fresnel devised an experiment whereby this interference could be produced without passing through apertures, which, according to the material hypothesis of light, might exert some attracting or diffracting effect on the passing beams. This consisted in allowing a beam of light from the focus of a lens to fall on two mirrors, very slightly inclined to each other, so as in fact to be almost in a straight line. To an eye viewing the reflections from the two mirrors obliquely, there will appear to be two bright lights very near together; and the interference will be apparent either when viewed directly in the eye, or when the beams are caught upon a screen, the effect being exactly the same as if the light actually proceeded from two contiguous points or openings. This is interference by reflection.

By means of a glass prism with a very oblique angle, a beam may be divided so as to travel in different paths; and the diverging cones of light will interfere if they meet again after having travelled unequal distances. This is interference by refraction.

918. There is also confirmatory evidence of the wave theory from what are known as phenomena of diffraction. Without going into details, the complete statement of which would involve mathematical technicalities, we may say that when a ray of light passes through a very minute opening, such for instance as a pin-hole in a sheet of tinfoil, or passes by a fine obstacle, such as a fine wire, the luminous waves bend outwards and inwards to some extent on each side of the geometrically straight path. Interference of waves in different phases thus takes place, and when the light which has passed in this way is received on a screen and carefully examined, coloured bands are seen, which are at once accounted for on the wave hypothesis of light. By drawing with a diamond, a number of minute lines very close together on a certain extent of any hard surface, we may produce coloured spectra showing a beautiful iridescence. These are owing to a similar cause. Polished steel thus treated, presents in different respects the splendid colours seen in the diamond itself. Some kinds of iridescent pearl owe their vivid colours and beauty to a minutely furrowed or striated surface, which may be seen by the microscope. This is proved by the fact that on taking an impression of the pearl on black wax, or on fusible metal, the iridescent colours are seen in the impression. There is simply a transference to the wax or metal, of the finely striated lines which produce the colours in the pearl. Fine fibres, such as the web of the spider, when a strong sunlight falls on them, present also iridescent colours.

Colours of Thin Plates.

683

These conditions are explicable on the principle of interference above described.

919. What are called the phenomena of thin plates, are also due to interference of luminous waves. The brilliant colours seen in the soap-bubble, or seen when a watch-glass or a lens, such as an eye of a pair of spectacles, is pressed on a piece of plate glass, or seen when a thin film of oil floats on clean water, are explained by the interference of the reflected luminous waves which proceed from the two surfaces of the thin transparent plate in each case. The first careful observation of these phenomena was made by Newton, and the iriscoloured rings, observed by pressing together two pieces of glass not quite flat, are generally known by the name of Newton's Rings. These are also seen in cracked ice, glass, or transparent crystals, and the iridescent colours of some kinds of opal are supposed to be owing to a similar cause.

Lastly, the ethereal hypothesis of light is remarkably elucidated and strengthened by a class of phenomena known as the polarization of light, which were at first regarded as destructive of the hypothesis; but which have now been completely reconciled with the theory.

POLARIZATION OF LIGHT.

920. If a beam of light, admitted by a hole in the shutter of a darkened room, be examined in any way, it is found to be symmetrical or of similar structure on all sides round the line or axis of transmission. If we let it fall on a plane mirror, the reflected image is equally bright and in all respects similar, on which ever side of the beam we present the mirror; the intensity depending only on the inclination of the mirror to the axis of the beam. To the naked eye this reflected beam appears to be precisely like the original one; and we should expect that, like the original, it is symmetrical on all sides round about. When, however, we try the effect of a second reflection on this reflected beam, we find that the second reflection is stronger when the second mirror is parallel to the first, than when its direction is across that of the first. The first reflected ray thus appears to have acquired sides, which property influences the behaviour of the beam in its subsequent course. Before explaining other methods by which this modification, or polarization, as it is called, of a beam is effected, we shall describe the simplest form of apparatus by which these experiments may be performed.

921. The Polariscope.-Twɔ rings, C, D (fig. 241), which fit on the

ends of a brass or pasteboard tube, T, carry two plane plate-glass mirrors, A and B, which are each mounted on an axle, so that they may be

inclined at any angle to the axis of the tube. Fixing the tube in an upright position, and allowing diffused daylight to fall on the mirror B, inclined to the axis of the tube, we find on inclining A to the axis in a direction parallel to that of B, that the reflection of daylight from B passes through the tube and is reflected again with little diminution by A; a bright round image of the further end of the tube is seen on looking into A. If now, keeping в unchanged in position, we slowly move the ring, C, round the axis of the tube, and watch the illuminated image in A of the far end of the tube, we find that this image gets darker and darker, until when the direction of A is right across that of B, there is more or less extinction of the light, according to the angle at which the mirrors, A and B, are inclined to the axis of the tube. When the lower mirror, B, is blackened with varnish on the back, and each is inclined to the vertical or the axis of the tube at an angle of about 33°, the extinction of the light is total when the directions of the mirrors are right across each other. If we keep turning A round the axis of the tube, we find that, after making another quarter of a turn, the second reflection is as strong as at first, while with three quarters of a complete turn, there is once more total extinction.

Fig. 241.

In place of B being a mirror of glass, we may use a surface of polished wood, ivory, leather, or any other non-metallic substance. The first reflector, B, is known as the polarizer of the light, and the second, A, as the analyser. Each reflecting surface has its own angle of maximum polarizing effect; and this is known as its polarizing angle. Sir David Brewster discovered that it bears a certain relation to the angle of refraction of the same substance; the relation being such, that when a surface is placed with respect to a beam of light at its polarizing angle, the reflected and the refracted rays are at right angles to each other.

The explanation of this phenomenon, which is possible under the undulatory theory alone, is that an ordinary beam of light con