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Question

A convex lens forms an image of a distant object at a distance of 20 cm from it. On keeping another lens in contact with the first, if the image is formed at a distance of 40/3 cm from the combination, then the focal length of the second lens is:
  1. -40 cm
  2. 13.33 cm
  3. -20 cm
  4. 40 cm

A
40 cm
B
13.33 cm
C
-20 cm
D
-40 cm
Solution
Verified by Toppr

Image of a distant object is formed at the focus.
The image due to first lens is formed at 20 cm. Hence, f1=20cm

The image due to combined lens is formed at 40/3 cm. Hence, feq=403

feq=f1f2f1+f2feq=20×f120+f1=403

3f1=40+2f1

f1=40cm

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