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?