\(\frac{\mu_2}{v}-\frac{\mu_1}{u}=\frac{\mu_2-\mu_1}{R}\)

\(u=\) infinity

\(\frac{3}{2 v}-0=\frac{1}{4 R}\)

\(v=6\,R\)

this serves as virtual object for refraction at second surface

\(u=+4\,R\)

\(\frac{\mu_2}{v}-\frac{\mu_1}{u}=\frac{\mu_2-\mu_1}{R}\)

\(\frac{5}{4 v}-\frac{3}{8 R}=\frac{1}{4 R}\)

\(V =2 R\)

distance from center is \(2 R+R=3 R\)