Question

Why the Newton’s rings are circular and centre of interference pattern (reflected) is dark?

Answer 1

In both air-wedge film and newton’s ring experiments, each fringe is the locus of points of equal thickness of the film. In newton’s ring experiment, the locus of points of equal thickness of air film lie on a circle with the point of contact of plano-convex lens and the glass plate as centre. So, the fringe are circular in nature and concentric..
For wedge shaped air film, the locus of points of equal thickness are straight, lines parallel to the wedge so fringes appear straight and parallel