Two coherent point sources $$S_1$$ and $$S_2$$ are separated by a small distance $$d$$ as shown in the figure. The fringes obtained on the screen will be

Fringes are formed at the locus of points where the path difference ($$\Delta x$$) between waves from $$S_1$$ and $$S_2$$ is constant.

$$\Delta x = |S_1Q - S_2Q| = \text{constant}$$

The common center of these circular fringes is the point $$P$$, where the line joining the sources $$S_1$$ and $$S_2$$ meets the screen.

Every point on the screen that is at an equal radial distance from the central point $$P$$ will have the same path difference, leading to a pattern of concentric circular fringes.