• # question_answer A double convex lens of focal length 6 cm is made of glass of refractive index 1.5. The radius of curvature of one surface is double that of the other surface. The value of smaller radius of curvature is: A)  6 cm             B)  4.5 cmC)    9 cm             D)    4 cm

$\frac{1}{f}=(\mu -1)\left\{ \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{1}}} \right\}$or $\frac{1}{6}(1.5-1)\left\{ \frac{1}{R}+\frac{1}{2R} \right\}$ $\Rightarrow$$R=4.5\,cm.$ Hence, the correction option is (b).