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NEET Sample Paper NEET Sample Test Paper-64

  • question_answer
    Three lenses \[{{L}_{1}},\,\,{{L}_{2}},\,\,{{L}_{3}}\] are placed co-axially as shown in figure. Focal lengths of lenses are / given 30 cm, 10 cm and 5 cm respectively. If a parallel beam of light falling on lens \[{{L}_{1}}\] emerging \[{{L}_{3}}\] as a convergent beam such that it converges at the focus of\[{{L}_{3}}\] . Distance between \[{{L}_{1}}\,and\,\,{{L}_{2}}\] will be      

    A) 40 cm              

    B) 30 cm         

    C) 2.0 cm             

    D) 10 cm           

    Correct Answer: C

    Solution :

    According to the problem, combination of \[{{L}_{1}}\,\,and\,\,{{L}_{2}}\] act a simple glass plate. Hence according to formula \[\frac{1}{F}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}-\frac{d}{{{f}_{1}}{{f}_{2}}}\] \[\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}-\frac{d}{{{f}_{1}}{{f}_{2}}}=0\,\,\Rightarrow \,\,\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}-\frac{d}{{{f}_{1}}{{f}_{2}}}\] \[\Rightarrow \,\,\frac{1}{30}-\frac{1}{10}=\frac{d}{30\times -10}\] \[\Rightarrow \,\,\,\frac{-20}{30\times 10}\,\,=\,\,-\frac{d}{30\times -10}\] \[\Rightarrow \,\,\,d=20\,cm\]


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