Q. A plano convex lenx $(\mu=1.5)$ has radius of curvature $20\, cm$. It is silvered on its plane surface. Find focal length after silvering:-
Ray Optics and Optical Instruments
Solution:
$\frac{-1}{f_{e q}}=\frac{2}{f_{\ell}}-\frac{1}{f_{m}}$
$f _{ m }=\infty ; \frac{1}{ f _{\ell}}=(1.5-1)\left\{\frac{1}{20}\right\}=\frac{1}{40}$
So, $\frac{1}{f_{\text{eg}}}=-\frac{2}{40} $
$\Rightarrow f_{\text{eq}}=-20 \,cm$
