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12th Class Physics Ray Optics Question Bank MCQs - Ray Optics and Optical Instruments

  • question_answer
    In a concave mirror, an object is placed at a distance \[{{x}_{1}}\] from the focus. Image if formed at a distance \[{{x}_{2}}\] from the focus. The focal length of the mirror is

    A) \[{{x}_{1}}{{x}_{2}}\]

    B) \[{{x}_{1}}{{x}_{2}}\]

    C) \[{{x}_{1}}/{{x}_{2}}\]

    D) None of these

    Correct Answer: D

    Solution :

    Option [d] is correct.
    Explanation: \[u=f+{{x}_{1}}\]
    \[v=f+{{x}_{2}}\]
    \[f=uv/\ (u+v)\]
    \[or,\ f=\frac{(f+{{x}_{1}})(f+{{x}_{2}})}{(f+{{x}_{1}})+(f+{{x}_{2}})}\]
    \[or\ {{f}_{2}}={{x}_{1}}{{x}_{2}}\]
    \[\therefore f\sqrt{{{x}_{1}}{{x}_{2}}}\]


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