State the use of echo by a bat, dolphin and fisherman.

Bats fly with speed much lower than the speed of sound. The sounds produced by the flying bats gets reflected back from an obstacle in front of it. By hearing the echo, the bats come to know, even in the dark, the location of the obstacle, so they turn from their path and fly safely without colliding with it. This process of detecting obstacle is called sound ranging.

Dolphins detect their enemy and obstacle by emitting the ultrasonic waves and hearing their echo. They use ultrasonic waves for hunting their prey.

A fisherman sends an ultrasonic pulse (a very high frequency vibrator) from a source and receives the pulse reflected from the shoal of fish in a detector. Hence, a fisherman use echoes to catch fishes.

The total time (t) of the two and fro journey of the pulse is recorded. The distance d of fish is then calculated by using the relation —

$\text{d} = \dfrac{Vt}{2} \\[0.5em]$

where, V is nearly 1400 ms^-1 (the speed of ultrasonic waves in sea water).