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Uttarakhand PMT Uttarakhand PMT Solved Paper-2010

  • question_answer
    A luminous object is placed 20 cm from surface of a convex mirror and plane mirror is set, so that virtual images formed in two mirrors coincide. If plane mirror is at a distance of 12 cm from object, then focal length of convex mirror is

    A)  5cm           

    B)  10cm

    C)  20 cm          

    D)  40 cm

    Correct Answer: A

    Solution :

     Here, \[u=-20cm,v=12+12-20=4cm\] \[\therefore \] \[\frac{1}{f}=\frac{1}{v}+\frac{1}{u}\] \[=\frac{1}{4}-\frac{1}{20}\] \[=\frac{5-1}{20}=\frac{4}{20}=\frac{1}{5}\] \[\therefore \] \[f=5cm\]


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