A student says that ultrasound waves are used in both sonar and foetal scanning because, in both cases, the time taken for an ultrasound pulse to return is measured. Explain how ultrasound is used in sonar to determine the distance to an object underwater, and describe ONE way in which foetal scanning is similar to sonar and ONE way in which its purpose is different.

Eduqas A-Level Physics — 5.2 Gravitational fields (A-Level only) · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Ultrasound waves have a frequency higher than the upper limit of human hearing (20 kHz). They can be emitted from a device, travel through a medium, reflect off a boundary, and be detected on return.

Model answer (5 marks)

Ultrasound pulses are emitted from a transducer and travel through the water until they reach an object. The pulse reflects off the object and returns to the transducer, where it is detected. The time taken for the round‑trip is measured; using the speed of sound in water, the distance to the object is calculated.

Foetal scanning is similar to sonar in that it also emits, reflects and detects ultrasound pulses, using the same basic principle of time‑of‑flight. The purpose differs: sonar is used to determine the distance to an underwater object, whereas foetal scanning uses the reflected echoes to construct an image of the foetus inside the womb.

Examiner tips

  • Use the word ‘time‑of‑flight’ to link pulse emission, reflection and detection; this shows understanding of the process.
  • Show the calculation step: distance = (speed of sound × time)/2 – this demonstrates application of the principle.
  • Mention the speed of sound in water (≈1500 m s⁻¹) to justify the calculation.
  • State clearly the two differences: distance measurement vs. imaging – this satisfies the ‘difference’ part of the question.

Common mistakes

  • Failing to mention the round‑trip time or dividing by two when calculating distance.
  • Confusing the purpose of sonar with that of foetal scanning, e.g. saying both are for imaging.
  • Using the wrong speed of sound (e.g. 340 m s⁻¹ for air) for underwater calculations.

Mark scheme (5 marks)

  1. Ultrasound pulses are emitted from a device and travel through the water
  2. The ultrasound reflects off the object (boundary) and returns to a detector
  3. The time taken for the pulse to return is measured and used to calculate the distance to the object
  4. Similarity: foetal scanning also uses the emission, reflection, and detection of ultrasound pulses (same process as sonar)
  5. Difference in purpose: sonar is used to determine distances to objects underwater, whereas foetal scanning is used to produce an image of a foetus in a womb

Key terms in this question

ultrasound waves · sonar · foetal scanning

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