Question

A thin equiconvex lens $(\mu =3/2)$ of focal length $10cm$ is cut and separated and a material of refractive index $3$ is filled between them. What is the focal length of the lens?

• A. $-10cm$
• B. $-10/4cm$
• C. $-10/3cm$
• D. None of these

Answer 1

Answer: (C)

$-10/3cm$

$\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$
$R=10cm$
$f^{\prime }=(3-1)(\frac{1}{-10}-\frac{1}{10})$
$f^{\prime }=\frac{-10}{4}$
$\frac{1}{f_{eq}}=\frac{1}{20}-\frac{4}{10}+\frac{1}{20}=\frac{2}{20}-\frac{4}{10}$
$f_{eq}=-10/3cm$.