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A convex lens is in contact with concave lens. The magnitude of the ratio of their focal length is 2/3. Their equivalent focal length is 30 cm. What are their individual focal lengths ?

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-10 cm, 15 cm

-15 cm, 10 cm

-75 cm, 50 cm

75 cm, 50 cm

answer is A.

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Detailed Solution

$\begin{array}{l} \frac{f_{1}}{f_{2}} = \frac{2}{3} . . . (i) \\ \frac{1}{f_{1}} - \frac{1}{f_{2}} = \frac{1}{30} . . . (ii) \\ Solving equations (i) and (ii) \\ f_{2} = - 15 cm (concave) \\ f_{1} = 10 cm (convex) \end{array}$

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