Question

Two convex lenses of focal length 20 cm each are placed coaxially with a separation of 60 cm between them. The distance of image formed of a distant object by the combination from rear lens is

• A. 80 cm
• B. 60 cm
• C. 40 cm
• D. 20 cm

Answer 1

The correct option is C 40 cm

Image of a distant object will form at focus (I) of the first lens $\left(L_1\right)$ as shown in the figure. This image will act as an object for the rear lens $\left(L_2\right)$. The image formed by the rear lens will be at a distance 'v' at point (I′) as shown in the figure.
For $L_2$,
u = –40 cm
f = 20 cm
Using lens formula,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$$\Rightarrow \frac{1}{v}+\frac{1}{40}=\frac{1}{20}$$\Rightarrow v=40cm$
Hence, the correct answer is option (3).