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JEE Main & Advanced Physics Wave Mechanics Question Bank Doppler's Effect

  • question_answer
    A whistle sends out 256 waves in a second. If the whistle approaches the observer with velocity 1/3 of the velocity of sound in air, the number of waves per second the observer will receive                [MP PET 1990; DPMT 2002]

    A)            384

    B)            192

    C)            300

    D)            200

    Correct Answer: A

    Solution :

               Wave number = \[\frac{1}{\lambda }\] but \[\frac{1}{\lambda '}=\frac{1}{\lambda }\left( \frac{v}{v-{{v}_{s}}} \right)\,\text{and}\ {{v}_{s}}=\frac{v}{3}\]                    \[\therefore \] (W.N.)? = (W.N.) \[\left( \frac{v}{v-v/3} \right)=256\times \frac{v}{2v/3}\]            = \[\frac{3}{2}\times 256=384\]


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