Interference Bands and their Applications1

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    THE formation of the interference bands, known as Newton's rings, when two slightly curved glass plates are pressed into contact, was illustrated by an acoustical analogue. A high-pressure flame B (Fig. 1) is sensitive to sounds which reach it in the direction EB, but is insensitive to similar sounds which reach it in the nearly perpendicular direction AB. A is a “bird-call,” giving a pure sound (inaudible) of wave-length (λ) equal to about 1 cm.; C and D are reflectors of perforated zinc. If C acts alone, the flame is visibly excited by the waves reflected from it, though by far the greater part of the energy is transmitted. If D, held parallel to C, be then brought into action, the result depends upon the interval between the two partial reflectors. The reflected sounds may co-operate, in which case the flame flares vigorously; or they may interfere, so that the flame recovers, and behaves as if no sonnd at all were falling upon it. The first effect occurs when the reflectors are close together, or are separated by any multiple of ½ √ 2. λ; the second when the interval is midway between those of the above-mentioned series, that is, when it coincides with an odd multiple of ¼ √ 2. λ. The factor √ 2 depends upon the obliquity of the reflection.

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    Interference Bands and their Applications1. Nature 48, 212–214 (1893) doi:10.1038/048212b0

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