BHU PMT BHU PMT (Mains) Solved Paper-2009

  • question_answer
    In a double slit experiment \[D=1\text{ }m,\text{ }d=0.2cm\] and \[\lambda =6000\,\overset{\text{o}}{\mathop{\text{A}}}\,\] . The distance of the point from the central maximum where intensity is 75% of that at the centre will be

    A)  0.01 mm             

    B)  0.03 mm

    C)  0.05 mm             

    D)  0.1 mm

    Correct Answer: C

    Solution :

    Resultant intensity \[I={{I}_{1}}+{{I}_{2}}+2\sqrt{{{I}_{1}}{{I}_{2}}}\cos \phi \] \[I=\frac{3{{I}_{0}}}{4}={{I}_{1}}+{{I}_{2}}+2{{I}_{1}}\cos \phi \] or            \[\frac{3}{4}\times 4{{I}_{1}}=2{{I}_{1}}(1+\cos \phi )\] or            \[\cos \phi =\frac{1}{2}=\cos \frac{\pi }{3}\] \[\therefore \]\[\phi =\frac{\pi }{3}\]and path differenced\[\frac{\lambda }{6}=\frac{xd}{D}\] \[\therefore \]    \[x=\frac{D\lambda }{6d}=\frac{1\times 6\times {{10}^{-7}}}{6\times 2\times {{10}^{-3}}}=0.05\,mm\]


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