For a plano convex lens $(\mu=1.5)$ has radius of curvature 10 cm . It is silvered on its plane surf

Question

For a plano convex lens $(\mu=1.5)$ has radius of curvature 10 cm . It is silvered on its plane surface. Find focal length after silvering.\

Professor PC Thomas Classes · Answer

(a) : $\frac{1}{f}=(\mu-1)\left[\frac{1}{R_{1}}-\frac{1}{R_{2}}ight]$\ $=(1.5-1)\left[\frac{1}{\infty}-\frac{1}{(-10)}ight]=0.5\left[\frac{1}{10}ight] \Rightarrow f=20 \mathrm{~cm}$\ When plane surface is silvered, $$ F=\frac{f}{2}=\frac{20}{2}=10 \mathrm{~cm} . $$