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

  • question_answer
    A rocket is going towards moon with a speed v. The astronaut in the rocket sends signals of frequency \[\nu \] towards the moon and receives them back on reflection from the moon. What will be the frequency of the signal received by the astronaut (Take v<<c)                             [RPMT 1996; DPMT 2000]

    A)            \[\frac{c}{c-v}\nu \]          

    B)            \[\frac{c}{c-2v}\nu \]

    C)            \[\frac{2v}{c}\nu \]           

    D)            \[\frac{2c}{v}\nu \]

    Correct Answer: B

    Solution :

               In this case, we can assume as if both the source and the observer are moving towards each other with speed v. Hence \[\nu '=\frac{c-{{u}_{o}}}{c-{{u}_{s}}}\nu =\frac{c-(-v)}{c-v}\nu =\frac{c+v}{c-v}\nu \] \[=\frac{(c+v)(c-v)}{{{(c-v)}^{2}}}\nu =\frac{{{c}^{2}}-{{v}^{2}}}{{{c}^{2}}+{{v}^{2}}-2vc}\nu \] Since v<<c, therefore \[\nu '=\frac{{{c}^{2}}}{{{c}^{2}}-2vc}=\frac{c}{c-2v}\nu \]


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