INTERFERENCE (from interfere, from OF. eat ref crir, to exchange blows. Fr. int, rft'rer, to interfere. from Lat. inter, ferire, to strike). In any medium eapable of transmitting waves. the effect at any point when two trains of waves are passing simultaneously is the sum of the effects which each wave by itself would pro dime, provided that the amplitudes of the waves are small compared with the wave-lengths; so that the resultant motion is the combination of the two waves. This is called 'interference.' The interference of two trains of waves lin the surface of water is often seen. The phenomenon of 'heats' in acoustics (q.v.) is due to interference. in all media, air, water, the ether, etc., can be made to interfere: and, conversely, if interference phenomena occur in connection with any physical action, it is evident that this action must be due to One of the simplest eases of interference is when two identical trains of wines are sent out from two vibrating sources close together. At any point whose distances from the two motives differ by half a wave-length. by three half mice lengths, or by any odd number of half wave lengths. the action of one train of s:l•es is per manently neutralized by that of the other. Whereas, at any point whose distances from the two sources are equal. or (tiller by a whole wave length, or by any number of whole wave•lengths, the action is twice what it would be, due to either train of waves by itself. Thus, let light of one definite wavelength, i.e. of a definite color, e.g. yellow, be passed through a narrow slit in an screen and fall upon a second opaque screen in which there are two other narrow slits parallel to the first. and at equal distances from it: if this pair of slits are close together, and if the light from them falls upon a distant Of some kind, there will be bands of colored light upon it, separated by dark bands where the ne t ion of the waves from one slit is neutralized by that of the waves from the other. The deviee
of having the two slits illuminated by light coin ing from the first slit is in order to secure two identieo/ sources of waves; for if the sources were to send nut different waves, or waVeS With out any definite phase-relation, there could be no permanent interference. In a perfectly similar manner waves in the air produced by whistles may be mac)• to give 'interference leinds' of silence and sound. The distance apart of the bands in any case evidently varies directly as the wavelength of the trains of waves. See LiollT.
Another case of interference is seen in the beautiful colors of soap-bubbles, of thin films of oil on water, etc. These colors are due to the fact !hat when white light falls upon the thin trans parent film, smile is reflected at the first surface and of the light that enters the film some is reflected at, the seemid surfacc. and the rest transmitted. There are, of course, a series of internal relleetioas: and looking at the film from either side the eye reeeives waves which have passed over paths of different lengths. If the difference in phase is such as to amount to half a \N.:lye-length or any odd number of half •ave lengths of waves or any definite wave-length, they will be absent from the light reeeived btthe eye. Put white light from any train of waves is absent will appear colored, having the color complementary to that of the absent waves.