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  4. Two thin lenses, one of focal length + 60 cm and the other of focal length -20 cm are put in contact, the combined focal length is:

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Q. Two thin lenses, one of focal length + 60 cm and the other of focal length -20 cm are put in contact, the combined focal length is:

VMMC MedicalVMMC Medical 2005

Solution:

Using the relation for combined focal length $ \frac{1}{f}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}=\frac{1}{60}+\frac{1}{(-20)} $ (Here: $ {{f}_{1}}=60\,cm\,{{f}_{2}}=-20\,cm $ ) $ \frac{1}{f}=\frac{1}{60}-\frac{1}{20}=\frac{1-3}{60}=\frac{-2}{60} $ $ f=-30\,cm $