8th Class Science Light Question Bank Light Formative Objective Problems

  • question_answer A fish is raising vertically to the surface of water in a lake uniformly at the rate of \[2m{{s}^{-1}}\] observes a bird diving vertically towards the water at a rate \[4m{{s}^{-1}}\] vertically above it. The refractive index of water is 4/3. The actual velocity of the bird is

    A)  3/2       

    B)  2/3    

    C)  4/3                       

    D)  3/4

    Correct Answer: A

    Solution :

      Velocity of fish is \[u=2m{{s}^{1}}\] Velocity of bird (w. r. t fish) is \[V=4m{{s}^{1}}\] Refractive index of water \[\mu =\frac{4}{3}\]actual velocity of bird\[=\frac{V-\mu }{\mu }=\frac{4-2}{\frac{4}{3}}=\frac{3}{4}\times 2=\frac{3}{2}\]               

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