question_answer

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\[[Take\text{ }g=10m/{{s}^{2}}]\]

A) 9m/s

B) 12m/s

C) 16m/s

D) 21.33 m/s

Correct Answer: C

Solution :

[c] Consider the activity A to B Applying \[{{v}^{2}}-{{u}^{2}}=2as\] \[{{v}^{2}}-{{0}^{2}}=2\times 10\times 7.2\] \[\Rightarrow v=12m/s\] The velocity of ball as perceived by fish is              \[v'{{=}_{w}}\mu \times v=\frac{4}{3}\times 12=16m/s\]