\(\frac{1}{f}=\frac{n_2-n_1}{R}=\frac{1.6-1}{5cm}\)

\(f=\frac{25}{3}cm\)

\(\frac{1}{s}+\frac{1}{s'}=\frac{1}{f}\)

\(s'=\frac{fs}{s-f}=\frac{\frac{25}{3}*15}{15-\frac{25}{3}}=\frac{75}{4}=18.75cm\)

\(AB=\frac{s+s'}{s}CL=\frac{15+18.75}{15}(7cm)=15.75cm\)