A ship uses sonar to map the depth of the ocean floor. Explain how sonar works to determine the depth of the water beneath the ship, and describe what happens to the ultrasound waves at the boundary between the water and the seabed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ships equipped with sonar devices emit ultrasound pulses downward from their hulls. The device both emits and detects ultrasound waves. The speed of sound in seawater is approximately 1500 m/s.
Model answer (5 marks)
Ultrasound pulses are emitted downwards from the ship’s sonar.
The pulses reach the water–seabed interface, where part of the energy is reflected back and part is transmitted into the seabed.
The reflected waves travel back to the ship, where a receiver detects them.
The time interval between emission and detection is recorded.
Using the known speed of sound in seawater (≈1500 m s⁻¹) the depth is calculated from depth = ½ speed × time.
The pulses reach the water–seabed interface, where part of the energy is reflected back and part is transmitted into the seabed.
The reflected waves travel back to the ship, where a receiver detects them.
The time interval between emission and detection is recorded.
Using the known speed of sound in seawater (≈1500 m s⁻¹) the depth is calculated from depth = ½ speed × time.
Examiner tips
- Use the command word ‘Explain’ – describe the process, not just state it. Show the key steps: emission, partial reflection, detection, time measurement, depth calculation. Include the speed of sound value and the ½ factor in the calculation explanation.
Common mistakes
- Forgetting that only part of the wave is reflected, or ignoring the transmitted part. Not including the ½ factor when relating time to depth. Using the wrong speed of sound or not stating it.
Mark scheme (5 marks)
- Ultrasound waves are emitted downward from the ship / sonar device
- At the boundary between water and the seabed, the waves are partially reflected back
- The remaining waves continue / are transmitted through the seabed (partial transmission)
- A receiver / detector (on the ship) detects the reflected waves and measures the time between emission and detection
- Because the speed of the waves is constant / known, the time measured can be used to calculate the depth of the water
Key terms in this question
ultrasound waves · boundary · sonar
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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