A bat navigates in complete darkness by emitting pulses of ultrasound and detecting the echoes that return from objects in its surroundings. Explain how the bat is able to locate an insect using this method, and explain why the bat cannot use visible light for this purpose in the same way.

OCR A-Level Physics B: Advancing Physics (H557) — 5.2 Circular motion and gravitational fields · Explain · 5 marks · View as Markdown

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

Bats are nocturnal mammals that hunt insects at night. They emit ultrasound pulses from their mouths or noses and their large ears detect the returning echoes. Bats can detect insect prey in total darkness with remarkable precision.

Model answer (5 marks)

The bat emits a short pulse of ultrasound. The pulse travels through the air and reaches the insect. At the interface between the insect and the surrounding air part of the sound is reflected back as an echo. The bat’s ears pick up this echo. Because the speed of sound in air is known, the bat can calculate the distance to the insect from the time interval between emission and reception of the echo. The pulses are at frequencies above 20 kHz, which is beyond human hearing but within the bat’s hearing range, and insects cannot hear them, giving the bat a hunting advantage. Visible light cannot be used in the same way in total darkness because there is no external source of light to reflect off the insect; the bat would need to emit light itself, which would be too weak and would be reflected by the surrounding darkness.

Examiner tips

  • Use the word ‘echo’ and ‘reflection’ to show understanding of wave behaviour.
  • Show the time–distance calculation: distance = ½ speed × time.
  • Mention the frequency range and its relevance to hearing.
  • Explain the lack of external light source in darkness.

Common mistakes

  • Confusing the speed of light with the speed of sound.
  • Forgetting that the bat emits the sound itself, not relying on external light.
  • Not linking the high frequency to the bat’s hearing advantage.

Mark scheme (5 marks)

  1. The ultrasound pulses travel outward and are partially reflected back at the boundary between the insect and the surrounding air (medium).
  2. The bat's ears detect the reflected (echo) pulses that return.
  3. The speed of the ultrasound waves is constant, so the time between emitting the pulse and receiving the echo tells the bat the distance to the insect.
  4. Ultrasound has a frequency above 20 kHz, which is above the upper limit of human hearing, but bats can detect it; this also means the pulses are not audible to insects, giving the bat a hunting advantage.
  5. Visible light cannot be used in the same way in total darkness because there is no external source of visible light to reflect off the insect; ultrasound is actively emitted by the bat itself so it does not rely on any external source.

Key terms in this question

echo

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