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  4. The magnification of an astronomical telescope is 10 and the focal length of eye-piece is 20 cm. The focal length of objective lens is :

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Q. The magnification of an astronomical telescope is 10 and the focal length of eye-piece is 20 cm. The focal length of objective lens is :

MGIMS WardhaMGIMS Wardha 2003

Solution:

Magnification of astronomical telescope $ m=\frac{{{f}_{0}}}{20} $ where $ {{f}_{o}}= $ focal length of objective lens $ {{f}_{e}}= $ focal length of eye piece $ \therefore $ $ 10=\frac{{{f}_{o}}}{20} $ $ \Rightarrow $ $ {{f}_{o}}=10\times 20 $ $ \Rightarrow $ $ {{f}_{o}}=200\,cm $