A total of 26 bright and 26 dark Newton's rings (not counting the dark spot at the center) are observed when 540 light falls normally on a

A total of 26 bright and 26 dark Newton's rings (not counting the dark spot at the center) are observed when 540 light falls normally on a planoconvex lens resting on a flat glass surface (see the figure in the textbook).How much thicker is the center than the edges?