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The interference is a phenomenon occurring in an area in which waves having the same frequency are superimposed. What happens, for instance, in a point of the water surface where waves arriving from two (or more) sources vibrating at the same frequency arrive?

So as to simplify the problem let us take into consideration the case in which there are only two sources, oscillating in a synchronized way. (In the wave parlance, we say that two waves having this characteristic are “in phase” with one another or that they are coherent). This situation can be easily created connecting two spherical terminals to the vibrating arm of the same ripple tank, so the motion of both terminals is identical.

If we were to observe the water surface from the top while illuminating the ripple tank with a grazing light level with the still water, we would see the crests of the waves illuminated and very clear, while the valleys would be in the shade and thus appear dark. The areas where the height of the still water is the one of water at rest will have an intermediate luminosity.

Inteference produced by two coherent sources of circular waves in the ripple tank. The images show fairly clearly that some areas of the liquid are still at zero level, that it at the level of the liquid at rest (excerpt from the 1976 movie “Electron interference” ).

Another example: a snapshot of the interference pattern produced by two coherent sources of circular waves within the ripple tank. Here the sources were obtained interposing an obstacle having two little openings along the path of plane waves, but the gist of the phenomenon stays the same. If we compare the regions close to both sources, where waves interfere with one another, with the one (e.g. toward the bottom of the picture) where the pattern produced by a single source dominates, we find that in the first there are areas of light and darkness more intense than the ones observed in the second. This reflects the fact that there are points where the interference produces higher crests and deeper valleys than the ones obtained by one source alone.

From the phenomena shown above we are led to conclude that in the region of the liquid where superposition of waves generated by two coherent sources occur, different points at the water surface behave in different ways generating typical geometrical configurations, called “interference patterns”. A typical element of interference patterns il the presence of areas where the surface appears practically unperturbed: these are the “nodal lines”, The point of the liquid surface enclosed within two adjacent nodal lines oscillate up and down shaping curvilinear wavefronts centered around a maximum amplitude (greater than the amplitude of the composing waves) which decreases to reaching zero value when reaching the nodal lines.