Question

The arrangement of two converging lenses is shown in the figure. The incident rays are parallel to the principle axis. What should be the value of $d$ so that a final real image of object is formed at infinity?

• A. $10\text{cm}$
• B. $30\text{cm}$
• C. $25\text{cm}$
• D. $50\text{cm}$

Answer 1

The correct option is B $30\text{cm}$

The final image will form at infinity if final rays are parallel to the principle axis. The incident rays refracted from the lens $L_1$ must converge at the first focus of lens $L_2$.


$\Rightarrow d=f_1+f_2=10+20=30\text{cm}$
Once the rays emerging from the lens $L_2$ become parallel to the principle axis, they will intersect at $\infty$ to produce a real image.

$\begin{array}{c}\text{Why this question}?\\ \text{Bottom line: First think that a real image}\\ \text{is formed at}\infty ,\text{then go backwards to}\\ \text{complete the ray diagram.}\end{array}$