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Image Formation by Convex Lens

an achromatic...

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an achromatic doublet of focal length 90 cm is to be made of two lens. The material of one of the lenses has 1.5 times the dispersive power of the other. The doublet is converging type. Find the focal length of each lens- (1) f1 = 20 cm and f2 = 30 cm (2) f1 = -20 cm and f2 = -45 cm (3) f1 = 30 cm and f2 = -45 cm (4) f1 = 15 cm and f2 = -20 cm

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