Chapter 24, Problem 72AP

A plano-convex lens (flat on one side, convex on the other) with index of refraction n rests with its curved side (radius of curvature R) on a flat glass surface of the same index of refraction with a film of index n_film between them. The lens is illuminated from above by light of wavelength λ. Show that the dark Newton rings that appear have radii of

$\text{r }\approx \sqrt{\text{mλ}R{\text{/n}}_{\text{film}}}$

where m is an integer.