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 an instant, when the ball is 12.8 m above the water surface, the fish sees the speed of ball as ______.

Options

• 9 ms⁻¹

• 12 ms⁻¹

• 16 ms⁻¹

• 21.33 ms⁻¹

Answer 1

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 16 ms⁻¹.

Explanation:

v = `sqrt(2gh)`

= `sqrt(2xx10xx7)`

= 12 ms⁻¹

In this case when eye is inside water,

x_app. = μx

∴ `(dx_"app.")/dt = mu.(dx)/dt`

or, v_app.μv = `4/3xx12`

= 16 ms⁻¹