A ball is dropped from a height of 20m above the surface of water in a lake. The refractive index of water is `4//3`. A fish inside the lake , in the line fall of the ball, is looking at the ball. At an instant when the balll is 12.8m above the water surface, the fish sees the speed of ball as .

Options

A. `9ms^(-1)`
B. `12ms^(-1)`
C. `16ms^(-1)`
D. `21.33ms^(-1)`

Answer

c.

Explanation

`V_("ball")^(2)=2xx10xx7.2rArrv=12=12ms^(-1)`
`X_("image of ball")=(4)/(3)X_("ball")`
`V_("image of ball")=(4)/(3)V_("ball")=(4)/(3)xx12=16ms^(-1)`