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J & K CET Medical J & K - CET Medical Solved Paper-2013

  • question_answer
    An object is 8 cm high. It is desired to form a real image 4 cm high at 60 cm from the mirror. The type of mirror needed with the focal length is

    A)  convex mirror with focal length\[f=40\text{ }cm\]

    B)  convex mirror with focal length \[f=20\text{ }cm\]

    C)  concave mirror with focal length\[f=-40\text{ }cm\]

    D)  concave mirror with focal length\[f=-20\text{ }cm\]

    Correct Answer: A

    Solution :

     \[M=\frac{I}{O}=\frac{v}{u}\] Given, \[O=8cm,\,I=4cm,v=60cm\] \[\therefore \] \[\frac{4}{8}=\frac{60}{u}\] \[\Rightarrow \] \[u-120cm\] \[\frac{1}{f}=\frac{1}{v}+\frac{1}{u}=\frac{1}{60}+\frac{1}{120}\] \[\Rightarrow \] \[f=40\,cm\]             (convex)


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