a tube pf length 30cm has its inner lateral surface blackened.two lenses of focal lengths 10 and 20 cm are fixed at ends of tube, for same beam intensities of spot on screen are I1 and I2 for arrangement when beam is focussed on 10cm side and I2 for other case then I1/I2 equals?1161/160.25

Dear student,

The correct ans is 1/16 ie c)

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Sagar Singh

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pls explain me how.....