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

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a

1.25 cm

b

2.5 cm

c

1.05 cm

d

2 cm

answer is B.

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Detailed Solution

Convex lens will form image I1, at its focus which acts like a virtual object for concave lens.Hence for concave lens u = + 4 cm, f = 20 cm. So by lens formula 1−20=1v−14⇒v=5 cm  i.e.,  distance   of  final   image  I3 from  concave  lens  v=5  cm  by  usingvu=IO⇒54=I2⇒I2=2.5  cm
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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