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NEET Sample Paper NEET Sample Test Paper-11

  • question_answer
    A lens having focal length and aperture of diameter d forms an image of intensity I. Aperture of diameter \[\frac{d}{2}\] in central region of lens is covered by a black paper. Focal length of lens and intensity of image now will be respectively. 

    A) \[f\,and\,\frac{1}{4}\]                

    B) \[\frac{3f}{4}\,and\frac{I}{2}\]

    C) \[f\,and\,\frac{3I}{4}\]               

    D) \[\frac{f}{2}\,and\,\frac{I}{2}\] 

    Correct Answer: C

    Solution :

     Intensity, \[I\propto {{A}^{2}}\] \[\frac{{{I}_{2}}}{{{I}_{1}}}={{\left[ \frac{{{A}_{2}}}{{{A}_{1}}} \right]}^{2}}=\frac{\pi {{r}^{2}}-\frac{\pi {{r}^{2}}}{4}}{\pi {{r}^{2}}}=\frac{3}{4}\] \[\Rightarrow \]\[{{I}_{2}}=\frac{3}{4}{{I}_{1}}\]


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