INTERPER'ENCE, a term employed to express the effect which rays of light, after being bent or diffracted, produce on each other. If the rays meet after diffraction, their light, when allowed to fall on a surface, will be divided into bars or stripes, alter nately light and dark, as is shown in the article diffraction (q.v.). This phenomenon has been made the touchstone of the two rival theories of light, the unclulatou and the emission. According to the former, it is thus explained: If two luminous waves simul taneously impel a molecule of ether, its motion will be the resultant of the original impulses; and if the two motions (as in the case of diffraction) be nearly in the same direction, the resultant will be nearly their sum; if opposite, their difference. Thus, when a particle has begun to undulate from the action of a luminous wave, and if, while in motion, another wave impinge upon it, the result will be increase of light. if the motion of the second wave conspire with that of the first; but a decrease. if they oppose each other; and total darkness, if, while opposing, they are equal in velocity. Let d be the distance corresponding to a complete period of vibration; then, if the second wave impinge upon the molecule after it has accomplished one or mor6 whole vibrations corresponding to the distances d, 2d, 31, etc., and has returned to its o:iginal
position, the two waves will evidently conspire together, and produce more violent motion; but if it impinge on the molecule, when the latter has only accomplished half a vibration, corresponding to distances id, Id, 4'd, etc., then the wave will oppose the particle's return to its original position; thus producing diminution of motion, or, if equal, rest. In the former case, the intensity of light is increased; in the latter, dimin ished; and if the undulations are of equal velocity, the light is doubled in the first ease, and destroyed in the seconch The emission theory totally fails to explain interference. In light of different colors, the value of d differs for each color, being least for violet, and greatest for red light. The principle of interference accounts in the most satisfac tory way for the colors of thin plates. the fringes that accompany shadows, etc.; and its explanation forms the most decisive reason yet known for adopting the undulatory in preference to the emission theory of light. See LIGHT.