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  4. 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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Q. 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?

NTA AbhyasNTA Abhyas 2020Ray Optics and Optical Instruments

Solution:

Let focal length of convex lens is + $f$ then focal length of concave lens would be $-\frac{3}{2}f$ .
From the given condition,
$\frac{1}{30}=\frac{1}{f}-\frac{2}{3 f}=\frac{1}{3 f}$
$\therefore \, f=10$ cm
Therefore, focal length of convex lens = + 10 cm and that of concave lens = -15 cm.