Question

Two convex lenses $A$ and $B$, each of focal length $30cm$ are separated by $30cm$, as shown in the figure. An object $O$ is placed at a distance of $40cm$ to the left of lens $A$.
What is the distance of the final image formed by this lens system?

• A. $120cm$ to right of lens $A$
• B. $90cm$ to right of lens $A$
• C. $22.5cm$ to right of lens $B$
• D. $45cm$ to right of lens $B$

Answer 1

Answer: (C)

$22.5cm$ to right of lens $B$

Object is at $40cm$ from lens $A$.
$\therefore$ for lens $A$
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{1}{v}=\frac{1}{30}-\frac{1}{40}=\frac{1}{120}$
$V=120cm$
$\therefore$ Image of lens $A$ is at $90cm$ at the right from lens $B$.
Now for lens $B$,
$\frac{1}{v}=\frac{1}{f}+\frac{1}{u}$
$\frac{1}{v}=\frac{1}{30}+\frac{1}{90}$
$v=22.5cm$
It will be at $22.5cm$ at the right of lens $B$.