Question

Image of an object at infinity is formed by a convex lens of focal length 30 cm such that the size of the image is 2 cm. If a concave lens of focal length 20 cm is placed in between the convex lens and the image, at a distance 26 cm from the convex lens, size of the new image is:

A
2.5 cm
B
2.0 cm
C
1.025 cm
D
1.05 cm
Solution
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Without the concave lens, image is formed at a distance 30 cm from the convex lens. When concave lens is placed, it serves as the object for the concave lens.

Hence, for concave lens, u=4 cm

Given, f=20 cm

From lens formula,

1v1u=1f

1v=14+120=420

v=5 cm

Magnification of lens, m=vu=54

Magnification of lens, m=hIho

hI=2×1.25=2.5 cm

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