Question

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, calculate the new size of the image.

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

Answer 1

Answer: (B)

Image formed by convex lens at $I_1$ will act as a virtual object
for concave lens. For concave lens
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$or\frac{1}{v}-\frac{1}{4}=\frac{1}{-20}$
$orv=5cm$
Magnification for concave lens
$m=\frac{v}{u}=\frac{5}{4}=1.25$
As size of the image at $I_1$, is 2 cm. Therefore, size of image at
$I_{2}willbe2\times 1.25=2.5cm.$