First slide

The lens

Question

A convex lens is in contact with concave lens. The magnitude of the ratio of their focal length is $\frac{2}{3}$ . Their equivalent focal length is 30 cm. What are their individual focal lengths

Moderate

A

-75, 50
B

-10, 15
C

75, 50
D

-15, 10

Solution

$\frac{f_{1}}{f_{2}} = \frac{2}{3} - - - - (i) \frac{1}{f_{1}} - \frac{1}{f_{2}} = \frac{1}{30} - - - - - (ii) Solving equation (i) and (ii) f_{2} = - 15 cm (Concave) f_{1} = 10 cm (Convex)$

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