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JEE Main & Advanced Physics Ray Optics Question Bank Critical Thinking

  • question_answer
    A fish rising vertically up towards the surface of water with speed 3 ms?1 observes a bird diving vertically down towards it with speed 9 ms?1. The actual velocity of bird is

    A)            4.5 ms?1

    B)            5. ms?1

    C)            3.0 ms?1

    D)            3.4 ms?1

    Correct Answer: A

    Solution :

                       Here optical distance between fish and the bird is                    \[s=y'+\mu y\]                    Differentiating w.r.t  t we get \[\frac{ds}{dt}=\frac{dy'}{dt}+\frac{\mu dy}{dt}\]            Þ \[9=3+\frac{4}{3}\frac{dy}{dt}\] Þ \[\frac{dy}{dt}=4.5\ m/\sec \]


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