The dispersive powers of the materials of the two lenses forming an achromatic doublet is 4 : 3. If the focal length of the combination is 60 cm, focal lengths of the two lenses are
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a
– 15 cm, 20 cm
b
15 cm , – 20 cm
c
20 cm, – 15 cm
d
20 cm, – 25 cm
answer is B.
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Detailed Solution
From equivalent focal length1F=1f1+1f2 1F=1f2(f2f1+1).............(1)From achromatic doubletw2w1=−f2f1..........2From 1 and 2f2=15cm, f1=−20cm