Question

Choose the correct answer from alternatives given.
The size of the image of an object, which is at infinity, as formed by a convex lens of focal length 30 cm is 2 cm. If a concave lens of focal length 20 cm is placed between the convex lens and the image at a distance of 26 cm from the convex lens, the new size of the image is

• A. 1.25 cm
• B. 2 cm
• C. 2.5 cm
• D. 1.05 cm

Answer 1

Answer: (C)

Convex lens forms the image at $I_1.I_1$ is at the second focus of convex lens. Size of $I_1=2cm$.

$I_1$ acts as virtual onject for concave lens. Concave lens forms the image of $I_1$ and $I_2$.

For concave lens,

$\frac{1}{v}-\frac{1}{4}=-\frac{1}{20}$ or $\frac{1}{v}=-\frac{1}{20}+\frac{1}{4}=\frac{4}{20}=\frac{1}{5}$

or $v=5cm=$ Distance of $I_2$ from concave lens.

$\because Magn$ ification $\frac{v}{u}=\frac{sizeofimage}{sizeofobject}=\frac{5}{4}$ or $\frac{sizeofimage}{2}=1.25$ or size of due $\to$ concave $\le$ ns $2.5cm$