Halliday, Resnick & Walker — Interference: Problem 27

27 A Newton’s rings apparatus is to be used to determine the radius of curvature of a lens (see Fig. 35-24 and Problem 25). The radii of the $n$th and $(n + 20)$th bright rings are found to be $0.162$ and $0.368 \text{ cm}$, respectively, in light of wavelength $420 \text{ nm}$. Calculate the radius of curvature of the lower surface of the lens.