In the Newton’s ring experiment, with source emitting $\lambda s$ 7000 & 6000 $\mathring{A}$, the $n^{th}$ dark ring of 7000 $\mathring{A}$ coincides with $(n+1)^{th}$ dark ring of 6000 $\mathring{A}$. If diameter of these overlapping rings is 0.5 cm, find radius of curvature of the plano-convex lens.