HANDBOOK OF ACOUSTICS - online book

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ON INTERFERENCE.
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remains at rest, for the condensation which enters at one branch and the rarefaction which simultaneously enters at the other, unite at B to neutralize one another's effects. When, however, the ends are held over alternate sectors, the sand is violently agitated, showing that they are in the same phase.
It is easy to illustrate the phenomenon of Interference, with no other apparatus than an ordinary tuning-fork. Let 0,0, fig. 74
Fig. 74.
represent the ends of the prongs of such a fork, looked down upon, as it stands upright. In the first place, let it be supposed, that these prongs are moving towards one another. In this case, the particles of air between the prongs will become more closely packed together, and consequently will crowd out both above and below, giving rise to condensations both in c and d. At the same time, in consequence of the inward swing of the prongs, the air particles to the left and right, sharing this movement, will be left wider apart than at first; that is, rarefactions will be formed at a and h. Now, let it be supposed that the prongs are making an outward journey; a partial vacuum will then be formed between them, and the air rushing in from without, will cause rarefactions at c and d ; while 0 and 0, pressing on the air at either side, will at the same time, give rise to condensations at a and b. Thus, we see, that as long as a tuning-fork is vibrating, four sets of waves are proceeding from it, two issuing in directly opposite directions from between the prongs, and two, also in directly opposite directions, at right