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 ______.

पर्याय

• 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(2"gh")=sqrt(2xx10xx7)`

= 12 ms^-1

In this case when eye is inside water,

x_app. = μx

∴ `("d"x_"app.")/"dt"=mu.("d"x)/"dt"`

or, v_app.μv = `4/3xx12`

= 16 ms^-1