JEE Main & Advanced Physics Wave Optics / तरंग प्रकाशिकी Question Bank Critical Thinking

  • question_answer
    A circular disc is placed in front of a narrow source. When the point of observation is at a distance of 1 meter from the disc, then the disc covers first HPZ. The intensity at this point is I0. The intensity at a point distance 25 cm from the disc will be

    A)            \[{{I}_{1}}=0.531{{I}_{0}}\]    

    B)            \[{{I}_{1}}=0.053{{I}_{0}}\]

    C)            \[{{I}_{1}}=53{{I}_{0}}\]   

    D)            \[{{I}_{1}}=5.03{{I}_{0}}\]

    Correct Answer: A

    Solution :

     \[{{I}_{0}}={{R}^{2}}=\frac{R_{2}^{2}}{4}\]             Number of HPZ covered by the disc at \[b=25\,cm\] \[{{n}_{1}}{{b}_{1}}={{n}_{2}}{{b}_{2}}\] \[{{n}_{2}}=\frac{{{n}_{1}}{{b}_{1}}}{{{b}_{2}}}+\frac{1\times 1}{0.25}=4\] Hence the intensity at this point is \[I={{{R}'}^{2}}={{\left( \frac{{{R}_{5}}}{2} \right)}^{2}}={{\left( \frac{{{R}_{5}}}{{{R}_{4}}}\times \frac{{{R}_{4}}}{{{R}_{3}}}\times \frac{{{R}_{3}}}{{{R}_{2}}} \right)}^{2}}\times {{\left( \frac{{{R}_{2}}}{2} \right)}^{2}}\] or \[1={{[0.9]}^{6}}\] \[{{I}_{1}}=0.531\,\,{{I}_{0}}\] Hence the correct answer will be (a).


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