NEET Sample Paper NEET Sample Test Paper-41

  • question_answer
    M. A convex lens of focal length f is placed somewhere in between an object and a screen. The distance between object and screen is x. if numerical value of magnification produced by lens is m, focal length of lens is:

    A) \[\frac{mx}{{{\left( m+1 \right)}^{2}}}\]                      

    B) \[\frac{mx}{{{\left( m-1 \right)}^{2}}}\]

    C) \[\frac{{{\left( m+1 \right)}^{2}}}{m}x\]                      

    D) \[\frac{{{\left( m-1 \right)}^{2}}}{m}x\]

    Correct Answer: A

    Solution :

    \[\operatorname{x}=u+v\]                              ?.(i) \[\operatorname{m}=\frac{f}{f-u}\,\,\,\,\,\Rightarrow \frac{f-v}{f}\] Image is real, magnification is negative \[-m=\frac{f}{f-u}\] \[\operatorname{u}=\left( \frac{m+1}{m} \right)\operatorname{f}\] \[-m=\frac{f-v}{\operatorname{f}}\] \[\operatorname{x}=\left[ \frac{m+1}{m} \right]f+\left( m+1 \right)f\Rightarrow f\left( m+1 \right)\left[ \frac{1}{m}+1 \right]\] \[\operatorname{f}=\frac{mx}{{{\left( m+1 \right)}^{2}}}\]


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