Question

If a fish moving in upward direction in water with $9\text{m/s}$ is observing a bird which is diving in air $12\text{m/s}$ then calculate speed of bird as seen by fish - $({\mu }_{\text{water}}=\frac{4}{3})$
(correct answer + 1, wrong answer - 0.25)

• A. $25\text{m/s}$
• B. $7\text{m/s}$
• C. $3\text{m/s}$
• D. $21\text{m/s}$

Answer 1

The correct option is A $25\text{m/s}$

$V_1=\frac{{\mu }_w}{{\mu }_a}{V}_0=\frac{4}{3}\times 12=16\text{m/s}$
Here, $V_1=$ actual speed of bird which is seen by fish at rest.
${\mu }_w=$ refractive index of water
${\mu }_a=$ refractive index of air
$V_0=$ speed of bird in air.
$\therefore$ Relative speed of bird as seen by fish in motion$=V_1+V_F=16+9=25\text{m/s}$.