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  4. A combination of convex and concave lenses has power 4D. If the convex lens has power 5D the focal length of the concave lens will be

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Q. A combination of convex and concave lenses has power 4D. If the convex lens has power 5D the focal length of the concave lens will be

JamiaJamia 2009

Solution:

Power of combination $ P={{P}_{1}}+{{P}_{2}} $ $ \Rightarrow $ $ 4+5+{{P}_{2}};{{P}_{2}}=4-5=-1\,D $ $ \therefore $ $ -1={{P}_{2}}=\frac{100}{f(cm)}\Rightarrow f=-100\,cm $