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Here above we describe nodal lines with yellow dashed lines. At this point, in order to connect directly to what we are going to see in the two slit experiment, we must imagine to observe the oscillation of the liquid produced by the interference of two sources on the surface of an imaginary wall which is dipped in water so as to intercept waves.

In the animation we identify the liquid by the blue color; the dashed line represent the level at rest. We see that the liquid oscillates in a fairly complicated way up and down around the rest line, but there are points (lying on the red vertical lines drawn to better emphasize them) which remain very close to the rest level during time. Such points correspond to the intersections of the nodal lines with the surface of the wall.

Now we imagine tu use ink as a liquid, so that the wall becomes coloured according to its wetting during oscillations.

The pattern observed after a given time is reported below.

In the region of the wall wetted by the oscillation of the liquid we observe a fairly regular pattern composed of many “hills” (the areas where the liquid oscillates up and down with respect to the rest level) saparated by points (nodal points) where the liquid oscillates very little. In the two-slit experiment we shall find a pattern very similar to this. Even if, in that case, the interfering waves are produced by two slits rather than by two vibrating tips, the gist of the matter is exactly the same.