# A bat navigates in complete darkness by emitting ultrasound pulses and detecting the returning echoes. Explain how the bat is able to locate a flying insect using this method, and explain why the hair cells inside the bat's cochlea are well suited to detecting the high frequencies used.

> OCR A-Level Physics A (H556) — 5.2 Circular motion and gravitational fields · Explain · 5 marks

> Bats emit ultrasound pulses with frequencies well above 20,000 Hz. The bat's cochlea contains specialised hair cells, each sensitive to a particular frequency, which trigger electrical impulses to the brain when they vibrate.

## Mark scheme (5 marks)

1. Ultrasound pulses are emitted by the bat and travel through the air until they reach the insect (a boundary between two media).
2. At the boundary between the insect and the surrounding air, the ultrasound waves are partially reflected back towards the bat.
3. The speed of the ultrasound waves is constant, so the bat can use the time between emitting the pulse and detecting the echo to determine the distance to the insect.
4. Ultrasound has a frequency above 20,000 Hz (above the upper limit of human hearing), so the bat's cochlea must contain hair cells tuned to these very high frequencies.
5. When the reflected ultrasound reaches the bat's ear, the hair cells tuned to that specific high frequency vibrate the most, triggering an electrical impulse to the brain, which interprets this as the location of the insect.

## Key terms

- [cochlea](https://www.gradenine.co.uk/glossary/cochlea)

## Related

- [Revision notes for OCR A-Level Physics A (H556)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-bat-navigates-in-complete-darkness-84b42285) · Published by Druglandscape Ltd.