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# The dispersive power of the materials of the two lenses are in the ratio 4:3. If the achromatic combination of these two lenses in contact is a convex lens of focal length 60 cm, then the focal length of the component lenses are 20 cm and -25 cm20 cm and +25 cm15 cm and +20 cm15 cm and -20 cm

A
20 cm and -25 cm
B
15 cm and +20 cm
C
20 cm and +25 cm
D
15 cm and -20 cm
Solution
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#### For an achromatic combination, f1w1+f2w2=0Also, the effective focal length of the combination will be:1f=1f1+1f2Solving these simultaneously and substituting the values, the obtained focal lengths are, fo=15 and fe=−20

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