A convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 40 cm. The combination behaves as a

A. convex lens of focal length 60 cm

B. convex lens of focal length 13.3 cm

C. concave lens of focal length 40 cm

D. convex lens of focal length 40 cm

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Answer: D. convex lens of focal length 40 cm

\(\frac1F=\frac1{20}-\frac1{40}=\frac1{40}\); positive, so converging, F = 40 cm.

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