Ultrasound waves are used in sonar and foetal scanning. Explain how ultrasound waves are used in sonar to determine the distance to an underwater object, and state ONE reason why ultrasound rather than sound within the range of human hearing is used for this purpose.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Ultrasound waves are emitted from the sonar transducer towards the target.
The waves travel through the water, strike the object and are reflected back.
The reflected echo is detected by the transducer.
The time interval between emission and detection is measured.
The distance to the object is calculated from the speed of sound in water and the measured time, taking into account that the wave travels to the object and back.
Ultrasound is used because its frequency is above the upper limit of human hearing, so it does not disturb people.
The waves travel through the water, strike the object and are reflected back.
The reflected echo is detected by the transducer.
The time interval between emission and detection is measured.
The distance to the object is calculated from the speed of sound in water and the measured time, taking into account that the wave travels to the object and back.
Ultrasound is used because its frequency is above the upper limit of human hearing, so it does not disturb people.
Examiner tips
- Use the word "emitted" and "reflected" to show the process. Show the time‑distance calculation (distance = ½ × speed × time). Mention the frequency being above 20 kHz to justify the choice of ultrasound.
Common mistakes
- Confusing the speed of sound in air with the speed in water. Forgetting to divide the time by two when calculating distance. Not stating why ultrasound is chosen (e.g. frequency above 20 kHz).
Mark scheme (5 marks)
- Ultrasound waves are emitted/transmitted from the device towards the object
- The ultrasound waves reflect off the object and are detected
- The time between emission and detection of the reflected wave is measured
- The distance is calculated using the wave velocity and the measured time (with awareness that the wave travels to the object and back)
- Ultrasound is used because its frequency is above the upper limit of human hearing (above 20 kHz) so it does not disturb humans / or because it can be directed in a narrow beam / or because it has a shorter wavelength giving better resolution
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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