Halliday, Resnick & Walker — Interference: Problem 25

25 Figure 35-24a shows a lens with radius of curvature R lying on a flat glass plate and illuminated from above by light with wavelength λ. Figure 35-24b (a photograph taken from above the lens) shows that circular interference fringes (known as 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 ≪ 1.