# A bat emits ultrasound pulses while flying towards a stationary wall. Explain why the frequency of the ultrasound detected by the bat, after reflection from the wall, is higher than the frequency emitted by the bat.

> IB DP Physics Standard Level (2023 syllabus) — C.5 Doppler effect · Explain · 4 marks

> Bats use echolocation to navigate and hunt. A bat flies directly towards a stationary wall, continuously emitting ultrasound pulses and detecting the reflected waves.

## Mark scheme (4 marks)

1. The bat is moving towards the wall, so successive wavefronts emitted by the bat are compressed in the direction of travel.
2. The wall therefore receives/detects a higher frequency than the emitted frequency (first Doppler shift).
3. The wall acts as a stationary source re-emitting/reflecting waves at this higher frequency, and the bat (observer) is moving towards the wall.
4. Because the bat (observer) moves towards the stationary wall (source of reflected waves), it encounters more wavefronts per unit time, so the detected frequency is further increased above the already-elevated reflected frequency — giving an overall detected frequency higher than the emitted frequency.

## Key terms

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

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](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-emits-ultrasound-pulses-while-688cb4e7) · Published by Druglandscape Ltd.