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Uttarakhand PMT Uttarakhand PMT Solved Paper-2007

  • question_answer
    When an object is moved along the axis of a lens, images three times the size of the object are obtained when the object is at 16 cm and at 8 cm respectively from the lens. The focal length and nature of the lens are

    A)  12 cm, concave

    B)  4 cm, concave

    C)  12 cm, convex 

    D)  4 cm, convex

    Correct Answer: C

    Solution :

     Since, magnified images are obtained at two different positions, so one image must be real and other must be virtual. \[3=\frac{f}{f-16}\]        (for virtual) \[-3=\frac{f}{f-8}\]         (for real) \[\Rightarrow \] \[f-16=-(f-8)\] \[\Rightarrow \] \[f-16=-f+8\] \[\Rightarrow \] \[2f=24\] \[\therefore \] \[f=+12\,cm\] (convex)


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