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 ball as?

Let the speed of the ball be v. Using the equation of motion, v² = u² + 2as, where u = 0, a = 10 m/s², and s = 7.2 m (20 m - 12.8 m).
v² = 0² + 2 × 10 × 7.2
v² = 144
v = 12 m/s
The apparent speed of the ball as seen by the fish is given by:
v' = (μ/μ')v
where μ is the refractive index of water (4/3) and μ' is the refractive index of air (1)
v' = (4/3) × 12 = 16 m/s