Question

A thin convex lens of focal length 20 cm is kept in contact with a plane mirror. The focal length of the combination will be :

• A. $5cm$
• B. $-10cm$
• C. $-20cm$
• D. $Infinite$

Answer 1

The correct option is B $-10cm$

$f_1=$ focal length of lens = $20cm$
$f_2=$ focal length of mirror = $\infty$
$Formula\Rightarrow$ $\frac{1}{f_{net}}=\frac{1}{f_{mirror}}-\frac{2}{f_{lens}}$
$\frac{1}{f_{net}}=\frac{1}{\infty }-\frac{2}{20}\Rightarrow 0-\frac{1}{10}$
$f_{net}=-10cm$