Question

An object is $1$ metre in front of a curved surface of a plano-convex lens whose flat surface is silvered. A real image is forme $120cm$ in front of the lens. What is the focal length of the lens?

• A. $9.1cm$
• B. $90.1cm$
• C. $109.1cm$
• D. none of these

Answer 1

The correct option is C $109.1cm$

$u=-100cmv=-120cmF=?\frac{1}{F}=\frac{1}{u}+\frac{1}{v}=\frac{1}{-100}+\frac{1}{-120}\therefore \frac{1}{F}=\frac{-11}{600}$

now, $\frac{1}{F}=\frac{2}{f_L}+\frac{2}{f_m}$

but $f_m=\infty$ (plane mirror)

$\therefore \frac{1}{F}=\frac{2}{f_L}\therefore f_L=2F=2\times \frac{600}{11}{f}_L=109.1cm$