Explain how ultrasound waves are used in sonar to determine the distance to an underwater object.

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).

A ship uses sonar to detect objects beneath the water. Ultrasound pulses are sent downwards from the hull of the ship and the returning signals are detected.

Model answer (5 marks)

1. Ultrasound pulses are emitted from the ship’s hull and travel through the water.
2. The pulses strike an underwater object and are reflected back.
3. The ship records the time interval between emission and reception of the echo.
4. Using the known speed of sound in water, the total distance travelled by the pulse is calculated.
5. The distance to the object is half this total distance, because the pulse travels to the object and back.

Examiner tips

  • Use the word ‘echo’ to show reflection. Show the time–distance calculation explicitly. Remember to divide by two for one‑way distance. Mention the speed of sound in water (≈1500 m s⁻¹).

Common mistakes

  • Forgetting to divide the total distance by two. Using the speed of light instead of the speed of sound. Not recognising that the time measured is for the round‑trip.]

Mark scheme (5 marks)

  1. Ultrasound waves are emitted/transmitted from the ship towards the underwater object
  2. The ultrasound waves reflect off the underwater object and return to the ship
  3. The time taken between emission and detection of the reflected wave is measured
  4. The speed of ultrasound in water is known, so the total distance travelled by the wave can be calculated
  5. The distance to the object is half the total distance calculated (because the wave travels to the object and back)

Key terms in this question

sonar · ultrasound waves

Related

More Gravitational fields (A-Level only) questions

▶ Try answering this question with AI marking (free) →