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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 ( $g = 10 m / s^{2}$ )

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$9 m s^{- 1}$

$12 m s^{- 1}$

$16 m s^{- 1}$

$21.33 m s^{- 1}$

answer is C.

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Detailed Solution

$ϑ_{b a l l} = \sqrt{2 g h} = \sqrt{2 \times 10 \times 7.2} = 12 m / s$

$h_{i b a l l} = \frac{4}{3} h_{b a l l}$

DOBS: $ϑ_{i b a l l} = \frac{4}{3} \times ϑ_{b a l l}$ $= \frac{4}{3} \times 12 = 16 m / s$

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