Q. A ball is dropped from a height of 20m above the surface of water in a lake. The refractive index of water is$\frac{4}{3}$. 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
A) $9\ {ms}^{- 1}$
B) $12\ {ms}^{- 1}$
C) $16\ {ms}^{- 1}$
D) $21.33\ {ms}^{- 1}$
Explanation
(c) $v = \ \sqrt{2gh} = \sqrt{2 \times 10 \times 7}$
$= 12\ {ms}^{- 1}$
In this case when eye is inside water,
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$x_{app.} = \ \mu x$
$\therefore\frac{dx_{app.}}{dt} = \mu.\frac{dx}{dt}$
Or $v_{app.} = \mu v = \frac{4}{3} \times 12 = 16\ ms^{- 1}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.