# A geologist uses a radioactive isotope to determine the age of a rock sample. The isotope has a half-life of 50 million years. The geologist measures the count rate from the sample and finds it is one-eighth of the count rate expected from a freshly formed rock of the same mass. Explain what is meant by the term 'half-life' and use it to determine how many years old the rock sample is.

> WJEC GCSE Physics (Wales) — 2.8 Half-life · Explain · 4 marks

## Mark scheme (4 marks)

1. Half-life is the time taken for half the radioactive nuclei (atoms) in a sample to decay
2. After one half-life the count rate is one-half of the original
3. Three half-lives are needed to reduce the count rate to one-eighth (½ → ¼ → ⅛)
4. The rock sample is 150 million years old (3 × 50 million years)

## Key terms

- [half-life](https://www.gradenine.co.uk/glossary/half-life)
- [count rate](https://www.gradenine.co.uk/glossary/count-rate)

## Related

- [Revision notes for WJEC GCSE Physics (Wales)](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-geologist-uses-a-radioactive-isotope-44428ec0) · Published by Druglandscape Ltd.