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CEE Kerala Engineering CEE Kerala Engineering Solved Paper-2006

  • question_answer
    The plane faces of two identical plano-convex lenses each having a focal length of 50 cm are placed against each other to form a usual biconvex lens. The distance from this lens combination at which an object must be placed to obtain a real, inverted image which has the same size as the object is:

    A)  50 cm                  

    B)         25 cm

    C)  100 cm                

    D)         40 cm

    E)  125cm

    Correct Answer: A

    Solution :

    The focal length of the combination is \[\frac{1}{F}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}\] Given,  \[{{f}_{1}}=50\,cm,{{f}_{2}}=50\,cm\] \[\therefore \]  \[\frac{1}{F}=\frac{1}{50}+\frac{1}{50}=\frac{2}{50}\] \[\Rightarrow \]               \[F=\frac{50}{2}=25\,cm\] Object when placed at centre of curvature forms a real, inverted image of same size as object\[=(2\times 25=50\text{ }cm)\].


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