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$\frac{4}{3}$ A fish 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 ($Take g = 10\,m/s^2$)

• A. $9 ms^{-1}$
• B. $12 ms^{-1}$
• C. $16 ms^{-1}$
• D. $21.33 ms^{-1}$

Answer 1

Answer: (C)

$16 ms^{-1}$

Answer 2

$v=\sqrt{2gh}=\sqrt{2\times10\times7}$
$12 ms^{-1}$
In this case when eye is inside water,
$x_{app}=\mu y x$
$\therefore\, \, \, \, \, \, \, \, \, \, \, \frac{dx_{app}}{dt}=\mu.\frac{dx}{dt}$
or $\, \, \, \, \, \, \, \, \, \, v_{app}=\mu v=\frac{4}{3}\times12=16ms^{-1}$