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

Answer

C

Explanation

Consider the activity A to B
Applying `v^(2)-u^(2)=2as`
`v^(2)-0^(2)=2xx10xx7.2 rArr v=12m//s`
The velocity of ball as perceived by fish is

`v.=""_(w)muxxv=(4)/(3)x12=16m//s`