A fish is rising up veritcally inside a pond with velocity `4 m//s,` and notices a bird, which is dividing downward and its velocity appear to be `16 m//s` (to fish). What is the real velocity of the diving bird, if refractive index of water is `4//3?`

Answer

C

Explanation

Given, `-dy/dt=4 cm//s`
Distance of bird as observed by fish
`Z=y+mux=y+4/3x`
`:. (-dZ)/dt=-dy/dt+4/3((-dx)/dt)……(i)`
Given, `(-dZ)/dt=16 cm//s`
Substituting in Eq. (i), we get
`-dx/dt=9 cm//s`