Volume 1

Science for all / edited by Robert Brown.

Date:: [1877?-1882?]

Credit: Science for all / edited by Robert Brown. Source: Wellcome Collection.

379/408 page 363

$in discussing these matters. Let w w', Fig. 2, repre- sent the water-level, and the wavy line a b a' h' the outline of a series of waves produced by droj^ping Fig. 2.—Illustrating Wave-Motion. a stone into water, then the elevations of water a a' are called crests, while the dej^ressions b b' are termed hollows or sinuses; and the length of a wave is measured from the to\) of one crest to the top of the next one, as, e.g., from a to a', or, what amounts to the same thing, the distance from the one hollow to the consecutive one, as from b to b'. We may now continue our comparison of wave-motion in the two media, water and luminiferous ether, or rather what we see in the former we may picture in the mind's eye as occurring in the latter, and thus explain some of the remarkable things in light that we have here to deal with. Instead of one, now throw at the same moment two stones on to the siu'face of the still water. Two systems of waves are produced, and they very soon meet at a point in the straight line which joins the centres of disturbance. Where the two sets of wave-rings meet, one of two things may haj^pen; either— (1) Crest may coincide with crest, and hollow with hollow, and give us crests and hollows of double height and depth at the place of meeting; or— (2) The crests of one system may coincide with the hollows of the other, and give us no wave at all at the place of meeting. How one wave may interfere with another, as in (1) and (2), is better seen after considei-ing the part an individual water-particle takes in the wave- motion. A cork placed on the surface of the water simply dances up and down when the wave passes; therefore, the water-particles which carry it simply dance up and down, and they do not travel along with the wave. The wave is consequently only a travelling form, and Ls pi'oduced by the communi- cation of motion from one set of water-pai-ticles to another. When a wave meets another, and is in com- plete accordance with it, as in (1), the wave result- ing is formed, as it were, by the supei'-position of one on the other;, for the water-particles would have been carried to a certain height and depi'essed to a certain ;lepth if only a single wave had passed the meeting- joint at a given moment; and when another passes :t the very same instant, a double force is exerted on the water-particles; they are carried to a double height and depressed nearly to a double depth. On the other hand, when one wave reaches the Fig. 3.—The Accordance of Waves. meeting-point half a wave-length before the other, or lags behind half a wave-length, crest is made to coincide with hollow, as in (2), and at the point of meeting a watei-particle is constrained to go up, and it is equally constrained to fall down. Between the two there is little, if any, motion at all. The first condition of things is represented by Fig. 3, where Fig. 4.—The Discordance of Waves. the waves a and b, being in accordance, produce the form {a + b). The second condition of things is represented by Fig. 4, wheie c and d, being in com- plete discordance, give us the result of their mutual action on each other (c + d)—i.e., no wave at all. Such phenomena go under the general name of Interference, and in whatever medium waves are produced, we may have such a mutual action of one set on another. In air, e.g., the sound-waves pro- ducing two notes slightly out of unison, will inter- fere with each other and give us beats—i.e., a succession, more or less rapid, of sound and silence. Again, in the luminiferous ether among the multi- tude of waves which produce white light, under certain conditions some may interfere with each other to the extinction of certain coloured rays, and thus • give us the colours of thin plates. The latter fact we must now enlarge upon. It may be stated generally that whenever a ray Fig. 5.—Interference of Light. of light is bent by passing from one substance into another, a portion of that ray is reflected back. The ray a b, Fig. .5, in passing from air into the denser medium plate-glass {g g), is partly reflected$

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