1. Tardigrade
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  4. 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.

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Q. 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.

IIT JEEIIT JEE 2003

Solution:

Image formed by convex lens at $I_1$ will act as a virtual object
for concave lens. For concave lens
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$or \frac{1}{v}-\frac{1}{4}=\frac{1}{-20}$
$or v=5 cm$
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
$I_2 will be 2 \times 1.25 = 2.5 cm.$

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