Question

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 the instant when the ball is 12.8 m above the surface, the fish would observe the speed of the ball as

(1) 9 ms^-1

(2) 12 ms^-1

(3) 16 ms^-1

(4) 21.33 ms^-1

Answer 1

(3) 16 ms^-1

In the Fig., B is the actual instantaneous position of the ball when it is at a height y above the surface of water. B₁ is the apparent position of the ball, as seen by the fish. The instantaneous apparent height OB₁ = y₁ . Let μ be the refractive index of water. We know that