A convex lens is in contact with concave lens. The magnitude of the ratio of their focal length is 23 . Their equivalent focal length is 30 cm. Their individual focal lengths are
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a
– 75 cm, 50 cm
b
– 10 cm, 15 cm
c
– 50 cm, 75 cm
d
– 15 cm, 10 cm
answer is D.
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Detailed Solution
From equivalent focal length 1F=1f1+1f2⇒12[f2f1+1] → 1 From achromatic doubletw2w1=−f2f1⇒f2f1=−23 → 2 From 2 and 1f1=−15cm, f2=10cm