Coherence and interference (constructive/destructive)

Two coherent point sources of light, ${\text{S}}_1$‍ and ${\text{S}}_2$‍, emit spherical light waves. Let's carefully survey the six points ($\bullet$‍) marked at various intersections of the waves, as shown below.

Let $I_n$‍ denote the intensity at point $n$‍.