The focal length of the field lens (which is an achromatic combination of two lenses) of telescope is 90 cm. The dispersive powers of the two lenses in the combination are 0.024 and 0.036. The focal lengths of two lenses are

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30 cm and 60 cm

30 cm and -45 cm

45 cm and 90 cm

15 cm and 45 cm

answer is B.

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Detailed Solution

For achromatic combinationf1f2=−ω1ω2=−0.0360.024=−32 and 1f1−1f2=190Solving above equations we get f1=30 cm, f2=−45 cm

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