Question

A double convex lens ( $R_1$ = $R_2$ = 100cm) having focal length equal to the focal length of a concave mirror. The radius of the concave mirror is

• A. 10 cm
• B. 20 cm
• C. 40 cm
• D. 15 cm

Answer 1

Answer: (B)

Focal length of convex lens $\frac{1}{f}=(\mu -1)\left[\frac{1}{R_1}-\frac{1}{R_2}\right]$ $R_1=10cm,$ $R_2=-10cm,$ $\mu =1.5$ $\frac{1}{f}=(1.5-1)\left[\frac{1}{10}-\frac{1}{-10}\right]$ $=0.5\left(\frac{2}{10}\right)$ $f=\frac{10}{2\times 0.5}$ $f=10cm$ Focal length of concave mirror = 10 cm $\therefore$ Radius of curvature $=2\times 10=20cm$