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  4. A convex lens of 40 cm focal length is combined with a concave lens of focal length 25 cm. The power of combination is:

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Q. A convex lens of 40 cm focal length is combined with a concave lens of focal length 25 cm. The power of combination is:

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Solution:

Power of combination is given by $ P=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}} $ Here, $ {{f}_{1}}=40\,cm=0.4\,m $ for convex, $ {{f}_{2}}=-25\,cm=0.25\,m $ for concave $ =\frac{1}{0.40}+\frac{1}{-0.25}=2.5\,D-4D=-1.5D $