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WB JEE Medical WB JEE Medical Solved Paper-2012

  • question_answer
    An object placed in front of a concave mirror  at a distance of \[x\,cm\]from the pole gives a 3 times magnified real image. If it is moved to a distance of \[(x+5)\,cm,\] the magnification of the image becomes 2. The focal length of the mirror is

    A)  15 cm           

    B)  20 cm

    C)  25 cm           

    D)  30 cm

    Correct Answer: D

    Solution :

     Given \[{{v}_{1}}=3x\]and \[{{v}_{2}}=2(x+5)\] So here, 1st case \[\frac{1}{-x}+\frac{1}{-3x}=\frac{1}{f}\] ?(i) \[\frac{1}{-(x+5)}+\frac{1}{-2(x+5)}=\frac{1}{f}\] ?(ii) On solving Eqs. (i) and (ii) we get \[f=30\,cm\]


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