A ship uses sonar to map the depth of the ocean floor. Explain how sonar waves are used to determine the depth of the ocean floor, and explain why the frequency of the sonar waves does not change as they travel from seawater into a denser layer of sediment on the ocean floor.

OCR A-Level Physics B: Advancing Physics (H557) — 5.3 Astrophysics and cosmology · Explain · 5 marks · View as Markdown

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

Sonar systems emit ultrasound waves from beneath a ship. These waves travel through seawater, reach the ocean floor, and return to a receiver on the ship. Scientists also study how the waves behave when they pass into different layers of material on the ocean floor.

Model answer (5 marks)

Sonar emits ultrasound waves that travel down through the water.
2. When the waves reach the ocean‑floor boundary they are partially reflected back to the ship.
3. The ship measures the time taken for the reflected wave to return.
4. Because the speed of sound in water is known, depth is calculated from depth = ½ × speed × time (greater time = greater depth).
5. The frequency of the sonar waves does not change when they enter a denser sediment layer because frequency is set by the source; only the wavelength and speed change, not the frequency.

Examiner tips

  • Use the 5 key points in order; show the depth formula with ½ factor.
  • Explain that frequency is a source property, not a medium property.

Common mistakes

  • Saying the frequency changes with depth; forgetting that only wavelength changes.
  • Omitting the ½ factor in the depth calculation or mis‑stating the speed as variable.

Mark scheme (5 marks)

  1. Ultrasound/sonar waves are emitted from the ship and travel down through the water
  2. Waves are (partially) reflected back when they reach the boundary/ocean floor
  3. The speed of the sonar waves is constant/known, so the time taken for the reflected wave to return to the receiver is measured
  4. The depth can be calculated because depth is proportional to the time taken (i.e. greater time = greater depth)
  5. Frequency does not change because frequency is a property of the source/emitter, not the medium; when entering a denser medium the wave slows down and the wavelength decreases but frequency remains constant

Key terms in this question

sonar · frequency

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