Keep the Path-difference guide and Combined-amplitude guide visible, and fix the probe at one screen height. The probe graph traces Source A, Source B, and their combined wave at that one point over time, while the pattern graph shows the long-run brightness at every screen height. A bright band is not one frozen crest in time. It is a position where the combined wave stays large cycle after cycle.
The key quantity is the total phase difference: how far out of step the two arrivals are when they meet. The Path-difference guide shows where part of it comes from. When you move the probe, r1 and r2 change, so the extra distance Delta r changes too. If Delta r is 0, one wavelength, two wavelengths, and so on, equal sources reinforce. If Delta r is about half a wavelength, one and a half wavelengths, and so on, equal sources arrive nearly opposite in phase and the screen becomes dark.
The source controls let you test which part of that phase difference changed. Source phase offset can turn the center from bright to dark without changing the geometry at all, because it adds phase directly. Changing Wavelength also leaves the geometry alone, but it changes how much phase each meter of path difference represents, so the fringes squeeze together or spread apart on the pattern graph.
One last check matters before you trust a dark band: destructive phase does not guarantee a perfect zero unless the source amplitudes are matched. If Source A and Source B are unequal, the combined-amplitude guide—the outline of how large the added wave can get—does not fully close. The same idea later reappears in standing waves, noise cancellation, and optical interference. Next, make a dark band by moving Probe height, return to the center and darken it again with Source phase offset alone, then unbalance the amplitudes and watch the null fill back in.
