Problem 5.88

At the crest of a spherical surface of a plano-convex lens there is a ground-off plane spot of radius $r_{0}=3.0 \mathrm{~mm}$ through which the lens comes into contact with a glass plate. The curvature radius of the lens's convex surface is equal to $R=150 \mathrm{~cm}$. Find the radius of the sixth bright ring when observed in reflected light with wavelength $\lambda=655 \mathrm{nm}$.