Interference

(-fér'ens.) [L. inter, between, and fe rio, I strike. ] (Optics.) If two similar waves start from the same place, at the same time, they increase each other's intensity, and the result is a wave of double light; but if one wave is half an undulation in advance of the other, the crest of one occupies the position of the hollow of the other, and the result is a dead level. If the intervals of starting are le than half a wave length, the result is a series of smaller waves, the magnitude of which depends upon the distance which one wave has in advance of the other. In the case of waves on the surface of water, this I. may easily be understood; and it has been found that similar phenomena obtain in the case of the ethereal vibrations which constitute the phenomena of light. [s. 488]