Figure a shows a lens with radius of curvature R lying on a flat glass plate and

Question:

Figure a shows a lens with radius of curvature R lying on a flat glass plate and illuminated from above by light with wavelength ? Figure b, (a photograph taken from above the lens) shows that circular interference fringes (called Newton's rings) appear, associated with the variable thickness d of the air film between the lens and the plate. Find the radii r of the interference maxima assuming r/R