Question

As shown in the figure, a combination of a thin plano concave lens and a thin plano convex lens is used to image an object placed at infinity The radius of curvature of both the lenses is $30 \,cm$ and refraction index of the material for both the lenses is $175$ Both the lenses are placed at distance of $40 \,cm$ from each other Due to the combination, the image of the object is formed at distance $x=$__ $cm$, from concave lens.

Answer 1

Correct Answer: 120

The correct answer is 120.
$\frac{1}{f_1}=(1.75-1)\left(-\frac{1}{30}\right)$
$\Rightarrow f_1=-40cm$
$\frac{1}{f_2}=(1.75-1)\left(\frac{1}{30}\right)\Rightarrow f_2=40cm$
Image from $L_1$ will be virtual and on the left of $L_1$ at focal length $40cm$. So the object for $L_2$ will be $80cm$ from $L_2$ which is $2f$. Final image is formed at $80cm$ from $L_2$ on the right.
So $x=120$