# Explain how astronomers use the Doppler effect to determine whether a distant galaxy is moving towards or away from Earth, and how the speed of that galaxy can be estimated from the observation.

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

> Astronomers analyse the light spectrum emitted by distant galaxies. Each element produces a characteristic set of spectral lines at known, fixed frequencies when measured in a laboratory on Earth.

## Mark scheme (4 marks)

1. The observed frequency (or wavelength) of a spectral line from the galaxy is compared to the known laboratory frequency (or wavelength) of the same line.
2. If the observed frequency is lower (wavelength longer) than the laboratory value — a red-shift — the galaxy is moving away from Earth.
3. If the observed frequency is higher (wavelength shorter) than the laboratory value — a blue-shift — the galaxy is moving towards Earth.
4. The fractional change in frequency (or wavelength) is proportional to the speed of the galaxy relative to Earth, so a larger shift corresponds to a greater speed.

## Key terms

- [Doppler effect](https://www.gradenine.co.uk/glossary/doppler-effect)

## 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/explain-how-astronomers-use-the-doppler-b2cab9d4) · Published by Druglandscape Ltd.