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JCECE Engineering JCECE Engineering Solved Paper-2012

  • question_answer
    The focal length of the objective of a terrestrial telescope is \[80\,\,cm\] and it is adjusted for parallel rays, then its power is\[20\]. If the focal length of erecting lens is \[20\,\,cm\], then full length of the telescope will be

    A) \[164\,\,cm\]                    

    B) \[124\,\,cm\]

    C) \[100\,\,cm\]                    

    D)  \[84\,\,cm\]

    Correct Answer: A

    Solution :

    Magnification for parallel rays,                 \[m=\frac{{{f}_{o}}}{{{f}_{e}}}\] \[\Rightarrow \]               \[20=\frac{80}{{{f}_{e}}}\] or            \[{{f}_{e}}=4\,\,cm\] If the focal length of erecting lens is\[20\,\,cm\], then the length of the telescope                 \[{{L}_{\infty }}={{f}_{o}}+4f+{{f}_{e}}\][where,\[f\] is the focal length of erecting lens]                 \[=80+4\times 20+4\]                 \[=80+80+4=164\,\,cm\]


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