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BHU PMT BHU PMT (Screening) Solved Paper-2010

  • question_answer
    An object is placed 21 cm in front of a concave mirror of radius of curvature 10cm. A glass slab of thickness 3 cm and                                                                                                                 \[\mu =1.5\] is then placed close to the mirror in the space between the object and the mirror. The position of final image formed is

    A)                                                                                                            \[-3.94\text{ }cm\]                

    B)  4.3 cm

    C)                                                                                                                    \[-4.93cm\]                

    D)  3.94cm

    Correct Answer: C

    Solution :

                     Due to glass slab, increase in path                                                                                          \[=(\mu -1)t=(1.5-1)3cm=1.5\,cm\]                                                                                                               \[\therefore\]                                                                                                       \[\mu '=-(21+1.5)cm\] or                                                                                                      \[\mu '=-22.5cm\] Now,                                                                       \[\frac{1}{-22.5}+\frac{1}{v}=\frac{1}{-5}\] Or                                                                               \[\frac{1}{v}=\frac{1}{22.5}-\frac{1}{5}\] Or                                                                             \[\frac{1}{v}=\frac{5-22.5}{22.5\times 5}\] Or                                                                                     \[v=\frac{22.5\times 5}{17.5}cm\] or                                                                                                          \[v=-\text{ }6.43\text{ }cm\] Geometrical distance                                                                                                    \[=-(6.43-1.5)\text{ }cm\]                                                                                                            \[=-\text{ }4.93\text{ }cm\]


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