Question

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

• A. $10cm$
• B. $20cm$
• C. $15cm$
• D. $25cm$

Answer 1

The correct option is A $10cm$

Equivalent power of combination,

$P_{eq}=2P_L+P_M$

$=2(\mu -1)\left(\frac{1}{R}\right)+0$

The required focal length,

$f=-\frac{1}{P_{eq}}=-\frac{R}{2(\mu -1)}=-\frac{10}{2(1.5-1)}=-10cm$