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JCECE Engineering JCECE Engineering Solved Paper-2015

  • question_answer
    A plano-convex glass lens \[({{\mu }_{g}}=3/2)\] of radius of curvature \[R=20\,\,cm\] is placed at a distance \['a'\] from a concave lens of focal length\[40\,\,cm\]. What should be the distance \['b'\] of a point object \[O\] from plano-convex lens so that the position of final image is independent of\[a\]?

    A) \[20\,\,cm\]                      

    B) \[60\,\,cm\]

    C) \[40\,\,cm\]                      

    D)  \[30\,\,cm\]

    Correct Answer: C

    Solution :

    Focal length of the plano-convex lens is                 \[\frac{1}{f}=({{\mu }_{g}}-1)\left( \frac{1}{20}-\frac{1}{\infty } \right)\]                 \[=\left( \frac{3}{2}-1 \right)\left( \frac{1}{20} \right)=\frac{1}{2}\times \frac{1}{20}\] \[\Rightarrow \]               \[f=40\,\,cm\]   If point object \[O\] is placed at a distance of \[40\,\,cm\] from the plano-convex lens, rays become parallel and final image is formed at second focus or \[40\,\,cm\] from concave lens which is independent of\[a\].


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