Optics Question 694

Question: 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

[IIT-JEE (Screening) 2005]

Options:

A) ? 75, 50

B) ? 10, 15

C) 75, 50

D) ? 15, 10

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Answer:

Correct Answer: D

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}=-15cm $ (Concave) $ f _{1}=10cm $ (Convex)



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