• # 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

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$=\frac{V-\mu }{\mu }=\frac{4-2}{\frac{4}{3}}=\frac{3}{4}\times 2=\frac{3}{2}$