A double-convex lens of power $\displaystyle 4$ D is manufactured from a glass of refractive index $\displaystyle 1 \cdot 5$. What is the radius of curvature of each face of this lens if both faces have the same radius of curvature?

\(\displaystyle \mathrm{P}=4 \mathrm{D}, \quad \mathrm{n}=1.5\) \(\displaystyle \mathrm{P}=\frac{1}{f}=(\mathrm{n}-1)\left(\frac{1}{\mathrm{R}_{1}}-\frac{1}{\mathrm{R}_{2}}\right)\) \(\displaystyle \mathrm{R}_{1}=\mathrm{R}, \mathrm{R}_{2}=-\mathrm{R}\) \(\displaystyle \mathrm{P}=\frac{1}{f}=(\mathrm{n}-1)\left(\frac{2}{\mathrm{R}}\right)\) \(\displaystyle 4=(1.5-1) \quad\left(\frac{2}{\mathrm{R}}\right)\) \(\displaystyle 4=\left(\frac{0.5 \times 2}{\mathrm{R}}\right)\) \(\displaystyle \mathrm{R}=\frac{1}{4}=0.25 \mathrm{~m}=25 \mathrm{~cm}\)