JCECE Medical JCECE Medical Solved Paper-2015

  • question_answer  A plano-convex glass lens \[({{\mu }_{g}}=3/2)\] of radius of curvature \[R=20\text{ }cm\]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 \[\frac{1}{f}=({{\mu }_{0}}-1)\left( \frac{1}{20}-\frac{1}{\infty } \right)\] \[=\left( \frac{3}{2}-1 \right)\left( \frac{1}{20} \right)=\frac{1}{2}\times \frac{1}{20}\] \[\Rightarrow \] \[t=40\,cm\] 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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