Sonar

Sonar is a technique in which sound is emitted in water and its reflection from a target is detected. The distance to the object is calculated from the transit time. In this case, a fishing boat locates a school of fish. The speed of sound in water is 1500 m/s, so the time to detect the reflecting waves for the target at a depth of 50 m is of the order of hundredths of a second, depending on the temperature and salinity of the water. Aquatic animals such as dolphins use the same principle. The analogous phenomenon in air is known as echolocation, and is used by bats.