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AMU Medical AMU Solved Paper-2012

  • question_answer
    An object is placed at a distance of 10 cm from co-axial combination of two lenses A and B contact. The combination forms a real image three times the size of the object. If lens B concave with a focal length of 30 cm. The nature and focal length of lens A is

    A)  convex, 12 cm                 

    B)  concave, 12 cm

    C)  convex, 6 cm                    

    D)  convex, 18 cm

    Correct Answer: C

    Solution :

                     Magnification \[m=\frac{v}{-u}=-3\]                                 v = 3u = 30 cm Focal length of co-axial combination                 \[\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}=\frac{1}{v}-\frac{1}{u}\]                 \[-\frac{1}{30}+\frac{1}{{{f}_{2}}}=\frac{1}{30}+\frac{1}{10}=\frac{4}{30}\]                 \[\frac{1}{{{f}_{2}}}=\frac{4+1}{30}=\frac{5}{30}\]                 \[{{f}_{2}}=\frac{30}{5}=6\,cm\] Hence, lens A is convex lens.


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