Problem 5.87

The convex surface of a plano-convex glass lens with curvature radius \(R=40 \mathrm{~cm}\) comes into contact with a glass plate. A certain ring observed in reflected light has a radius \(r=2.5 \mathrm{~mm}\). Watching the given ring, the lens was gradually removed from the plate by a distance \(\Delta h=5.0 \mu \mathrm{m} .\) What has the radius of that ring become equal to?