# A bat emits ultrasonic pulses and detects echoes reflected from a moth flying directly towards the bat at constant velocity. Explain why the frequency of the echo detected by the bat is higher than the frequency emitted by the bat.

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

## Mark scheme (4 marks)

1. The moth acts as a moving observer approaching the source (bat), so it encounters wavefronts at a higher rate than if it were stationary.
2. The moth therefore reflects (re-emits) pulses at a higher frequency than the original emitted frequency.
3. The moth now acts as a moving source approaching the bat, so the bat detects wavefronts that are compressed (shorter wavelength), increasing the detected frequency further.
4. The overall detected frequency is higher than the emitted frequency because the Doppler effect acts twice — once for the moth as a moving observer and once for the moth as a moving source.

## Key terms

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

## Related

- [Revision notes for IB DP Physics Higher 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-ultrasonic-pulses-and-3b4c3f8c) · Published by Druglandscape Ltd.