Sonar and foetal scanning both use ultrasound waves. Explain why ultrasound waves are used in these applications rather than sound waves that humans can hear, and describe what happens to the ultrasound waves in each application to produce useful information.

Ultrasound waves have a frequency above 20 kHz, the upper limit of human hearing, so they are inaudible and do not disturb the user.

They are chosen because the high frequency gives a short wavelength, which allows good resolution, and the energy is low enough that it does not damage biological tissue, making it safe for medical use.

In sonar, a transducer emits a burst of ultrasound into water. The waves travel until they encounter an object such as the sea‑bed, are reflected back, and are received by the same or another transducer. The time taken for the echo to return is measured and, using the speed of sound in water, the distance to the object is calculated.

In foetal scanning, a probe emits ultrasound into the abdomen. The waves pass through the body and are reflected at interfaces where the acoustic impedance changes (e.g. between foetal tissue and amniotic fluid). The reflected echoes are detected, their time delays are converted into depth information, and the signals are processed to form a two‑ or three‑dimensional image of the foetus.