Haryana PMT Haryana PMT Solved Paper-2003

  • question_answer
    In double slit experiment, the angular width of interference fringes with sodium light \[(\lambda =5890\overset{\text{o}}{\mathop{\text{A}}}\,)\] is \[0.20{}^\circ \]. The change in wavelength required to increase the angular. width by 10% will be :

    A) zero                                      

    B)  increased by \[6479\overset{\text{o}}{\mathop{\text{A}}}\,\]

    C)  increased by \[589\overset{\text{o}}{\mathop{\text{A}}}\,\]

    D)  decreased by \[589\overset{\text{o}}{\mathop{\text{A}}}\,\]

    Correct Answer: C

    Solution :

                    Angular fringe width \[\theta =\frac{\beta }{D}=\frac{\lambda }{2d}\] i.e., \[\theta \propto \lambda \] or    \[\frac{\theta }{\theta }=\frac{\lambda }{\lambda }\]  or  \[\frac{\theta -\theta }{\theta }=\frac{\lambda -\lambda }{\lambda }\] Hence, percentage increase in wavelength = percentage increase in an angular fringe width =10%                            : or increase in wavelength                 \[=\frac{10}{100}\times 5890\overset{\text{o}}{\mathop{\text{A}}}\,\]                 \[=589\overset{\text{o}}{\mathop{\text{A}}}\,\]


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