A ball is dropped from a height of 20 m above the surface of water in a lake. The refractive index of water is 4.3. A fish inside the lake, in the line of fall of the ball, is looking at the ball. At an instant, when the ball is 12.8 m above the water surface, the fish sees the speed of the ball as `[Take _(g)=10m/s^(2).]`

Options

A. 9 m/s
B. 12 m/s
C. 16 m/s
D. 21.33 m/s

Answer

C

Explanation

`v_(2) - u_(2) = 2"as" , v_(2) = 2(g) (7.2), v = 12 m//s`

`X_(b//f) = mux`
`(dX_(b//f))/(dt) = mu.(df)/(dt) implies (4)/(3) (12 m//s) implies 16 m//s`