INTERFERENCE, in physics, a name ap plied to the mutual action of "waves" of any kind upon one another, a phenomenon by which under certain condition the vibrations and their effects are increased, diminished or neutralized. A wave, obiter dicta, in the sense of the word as here employed, should convey the idea of a °forward-moving form of motion" (like waves in a wheat field), and should not convey—in a way productive of much error in the past — a notion of a "forward-moving mass of mat ter.' (Consult Mach, 'Popular Science Lec tures' 1910). Motion, finally is a change of place; a transformation, changing of forms in volving displacements; a new arrangement of parts in space is the main result of motion.
As a word, interference was formed on the false analogy of such words as "difference"- it was derived from interfere, which in the 1.-;tin original ferire conveyed the idea of a horse striking one foot or ankle against an opposite foot or ankle, while moving along. Thomas Young in
The kinds of interference that are of the greatest practical importance in physics are those which occur among sound waves or among waves of light. The phenomena in these
cases are ultimately of the same general sort as those observed upon the surface of water. A familiar result of the interference of sound waves is the production of "beats" when two or more trains of waves, having but slightly dif ferent wave-length, come together while the two are moving in' nearly the same direction. This phenomenon is exhaustively treated in Helmholtz's 'Sensations of Tone,' and it also receives a more or less adequate treatment in all of the better works on physics.
The more familiar of the interference phe nomena that are afforded by light are those which are observed in connection with soap bubbles and with very thin plates of transparent solids. Light, upon striking the soap bubble or the thin plate, is reflected toward the eye from both surfaces, and the trains of light waves that reach the eye from these two sources, since they have a slight difference of phase (which varies, moreover, from point to point of the bubble or the plate), interfere with one another so as to produce effects that are often very beautiful and striking. A soap bubble, when viewed by monochromatic light, often appears to be covered with dark striae; the dark lines being due to the fact that at the points that appear dark the two trains of light waves, coming respectively from the inner and outer surfaces of the soap bubble, nearly or completely neutralize each other. When the bubble is viewed by white light, we do not commonly see the dark stria, their places being taken by bands of color. This is because the different colors that compose white light have different wave-lengths, so that at any given point in the bubble only a portion of the colors are destroyed by interference, leaving the re maining constituents of the white light to pro duce their full chromatic effect upon the eye. For further information consult Cornish, V., 'Waves of the Sea and other Water Waves); the same author's 'Waves of Sand and Snow' (both Chicago 1915); Michelson,