$\begin{aligned} & \mathrm{m}=\frac{\mathrm{n}_1}{\mathrm{n}_2} \frac{\mathrm{v}}{\mathrm{u}} \\ & \frac{\mathrm{n}_2}{\mathrm{v}}-\frac{\mathrm{n}_1}{\mathrm{u}}=\frac{\mathrm{n}_2-\mathrm{n}_1}{\mathrm{R}} \\ & \frac{1.4}{\mathrm{v}}+\frac{1}{4 \mathrm{R}}=\frac{1.4-1}{\mathrm{R}} \\ & \frac{1.4}{\mathrm{v}}=\frac{4}{10 \mathrm{R}}-\frac{1}{4 \mathrm{R}} \\ & \mathrm{v}=\frac{56 \mathrm{R}}{6}=+\frac{28}{3} \mathrm{R} \\ & \mathrm{m}=\frac{1}{1.4} \cdot \frac{28 \mathrm{R} / 3}{(-4 \mathrm{R})}=-1.67\end{aligned}$