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Chhattisgarh PMT Chhattisgarh PMT Solved Paper-2006

  • question_answer
    The plane faces of two identical plano-convex lenses, each having focal length of 40 cm, are placed against each other to form a common convex lens. The distance from this lens at which an object must be placed to obtain a real inverted image with magnification equal to unity is

    A)  80cm                                   

    B)  40cm

    C)  20cm                                   

    D)  160cm

    Correct Answer: B

    Solution :

    Focal length of the combination lens, \[\frac{1}{f}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}=\frac{1}{40}+\frac{1}{40}\]                 \[\therefore \]  \[f=20cm\] Given,  \[m=1\]                 \[m=\frac{f}{u-f}\]                 \[1=\frac{20}{u-20}\]                 \[u-20=20\]                 \[u=40cm\]


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