College Physics 10th Edition by Raymond Serway,Chris Vuille

Exercise 78

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 . Show that the dark Newton rings that appear have radii of
where m is an integer.