NEET Sample Paper NEET Sample Test Paper-49

  • question_answer
    A lens is placed between a source of light and a wall. It forms images of area \[{{A}_{1}}\] and \[{{A}_{2}}\] on the wall, for its two different positions. The area of the source of light is-

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

    B) \[\frac{{{A}_{1}}+{{A}_{2}}}{2}\]

    C) \[{{\left( \frac{\sqrt{{{A}_{1}}}+\sqrt{{{A}_{2}}}}{2} \right)}^{2}}\]  

    D) None of these

    Correct Answer: A

    Solution :

    \[{{m}_{{{A}_{1}}}}=\frac{{{A}_{1}}}{{{O}_{1}}}\,\,\,\,\,\,\,\,{{m}_{{{A}_{2}}}}\,\,=\,\,\frac{{{A}_{2}}}{{{O}_{1}}}\,\,\,\,\,\,\,\,\,\,({{O}_{1}}={{O}_{2}})\,\,\,\,\,\] \[{{m}_{{{A}_{1}}}}\times \,\,{{m}_{{{A}_{2}}}}\,=\,\,\left( \frac{{{A}_{1}}}{{{O}_{1}}} \right)\,\,\,\,\left( \frac{{{A}_{2}}}{{{O}_{1}}} \right)\,\,\,\,\,={{m}_{1}}^{2}{{m}_{2}}^{2}\,\,but\,\,{{m}_{1}}{{m}_{2}}\,=\,1\,\,\]\[so\,\,\,\frac{{{A}_{1}}}{{{O}_{1}}}\,\times \,\frac{{{A}_{2}}}{{{O}_{1}}}\,\,=\,\,1\,\,\Rightarrow \,\,{{O}_{1}}^{2}\,\,=\,\,{{A}_{1}}{{A}_{2}}\] Hence \[{{O}_{1}}={{O}_{2}}\,=\,\sqrt{{{A}_{1}}{{A}_{2}}}\]


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