\(\frac{1}{f}=\frac{n_2-n_1}{n_1}(\frac{1}{R_1}-\frac{1}{R_2})\)

in air \(\frac{1}{30}=\frac{1.5-1}{1}(\frac{1}{R_1}-\frac{1}{R_2})\)

in the liquid \(\frac{1}{-188}=\frac{1.5-n_L}{n_L}(\frac{1}{R_1}-\frac{1}{R_2})\)

\(\frac{-188}{30}=\frac{0.5n_L}{1.5-n_L}\)

\(n_L=1.62\)