Interference

Interference, in Physical Science, is a term which refers to a very general class of phenomena depending on the co-existence at one place of two different sets of waves, undulations, or vibrations. Its essential character is well illustrated by the mingling of two sets of ripples produced in any way (such as by the dropping in of stones) on the otherwise smooth surface of a sheet of water. Where crest meets crest, and trough meets trough, there the resultant disturbance is increased; but where crest meets trough, and trough meets crest, the disturbance will be diminished, and even annihilated should the mingling ripples be equal to begin with. In such a case we can observe the interference of individual waves. Now, wherever we have wave-motion, in the wide dynamic sense of the term, there we may have interference-phenomena showing themselves. But if, as in the case of the propagation of sound, light, and electrical waves, the undulations are too small, or of a character too peculiar to be individually observed or felt by any of our senses, we cannot hope to have evidence of interference-phenomena unless there is a steady succession of two trains of waves reproducing the same phenomenon at the same place for an indefinite time. Thus, two different rays of light will not in general produce evident interference-phenomena. It is only when they have been brought from the same original source, and made to pursue slightly different paths, that the optical effects of interference are possible. As a simple illustration, take Grimaldi's experiment as modified by Dr Thomas Young (1804), to whom we owe the discovery of the principle of interference and its application to optical phenomena. A ray of light, which for simplicity we shall regard as homogeneous—that is, of one wave-length and colour—is introduced into a darkened chamber through two minute apertures very close together. The two similar divergent rays of light so produced will interfere, and the result, as shown on a screen placed a short distance in front of the apertures, will be a series of bright bands separated by dark spaces. The central bright band, every point of which is equidistant from the apertures, is produced by the superposition of two rays, crest falling with crest, and trough with trough. The next bright band on either side is the locus of all points whose distances from the two apertures differ by a wave-length of light, so that still crest falls with crest, and trough with trough. But at the points that lie in the centre of the intermediate dark space the two rays meet so that crest falls with trough, and trough with crest, and thus produce darkness instead of brightness. The general law is that darkness is produced when the portions of the two interfering rays that coexist at one point were in the original single ray distant from each other by an odd number of half wave-lengths; and that brightness is produced when this distance is an even multiple of a half wave-length. Theoretically an indefinite number of interference bands should be visible; but practically this is not so. The chief reason for the gradual fading of the further bands is the difficulty of obtaining sufficiently pure homogeneous light. If the light is ordinary sunlight it will be found impossible to get really dark spaces, since in this case the component rays, being of different wave-lengths, cannot interfere in the same way. Thus, if the red rays interfere so as to annihilate each other, the blue rays will not do so, but may on the contrary interfere to intensify each other. Hence arise the coloured bands always to be seen when interference-phenomena are produced with non-homogeneous light. Amongst other optical illustrations of the principle of interference we may mention the corona round the sun and moon when they are seen through a fleecy cloud, the spurious bows that fringe the primary rainbow, the colours of soap films and thin plates generally, the colours of mother-of-pearl and diffraction gratings (see SPECTRUM), Newton's rings, and, as a simple experiment, the appearance of a candle or lamp flame when looked at through a fine cambric handkerchief. The phenomenon of spring and neap tides (see TIDES) is another case of interference; so also are shadows, both light-shadows and sound-shadows. Moreover, Dr Hertz of Carlsruhe has taught us how to obtain and measure the interference of electro-magnetic waves. See MAGNETISM.