Question

A light beam parallel to axis is incident on the system of four convex lenses $A,B,C$ and $D$. Focal lengths of $A,B,C$ and $D$ are $30cm,10cm,30cm$ and $10cm$, respectively as shown. Here, fixed distance $BC=20cm$. What should be the distance between the lens $A$ and lens $D$ so that after refractions, rays will be parallel to axis in region I, III and V?

• A. $40cm$
• B. $100cm$
• C. $80cm$
• D. $20cm$

Answer 1

Answer: (B)

$100cm$

The light rays are coming from infinity and are incident on lens A parallel to the axis. It will form point image at $F$ i.e. on focal point.
This $F$ will act as source of light for lens $B$ after refraction $B$, the emergent ray will parallel to axis of lens. Similarly, after refraction from $C$, theny will converge at $F$. This $F$ will act as a source. After refraction from $D$, the light rays will emerged parallel to the axes of lens.
Now, from figure
$AD=30+10+20+30+10$
$=100cm$