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JEE Main & Advanced Physics Ray Optics Question Bank Self Evaluation Test - Ray Optics and Optical Instruments

  • question_answer
    When an object is placed at a distance of 25 cm from a mirror, the magnification is\[{{m}_{1}}\]. The object is moved 15cm further away with respect to the earlier position, and the magnification becomes  If \[{{m}_{1}}/{{m}_{2}}=4,\] the focal length of the mirror is:

    A) 10 cm

    B) 30 cm

    C) 15 cm

    D) 20 cm

    Correct Answer: D

    Solution :

    [d] \[m=-\frac{v}{u}-\left( \frac{f}{u-f} \right)\] \[\text{Now }{{\text{m}}_{1}}=-\left( \frac{f}{25-f} \right)\]         ??(i) \[\text{and }{{m}_{2}}=-\left( \frac{f}{40-f} \right)\]        ??(ii) \[\therefore \frac{{{m}_{1}}}{{{m}_{2}}}=\frac{40-f}{25-f}\text{ or }4=\frac{40-f}{25-f}\text{ or f=20}\,\text{cm}\text{.}\]


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