# A scientist is investigating the emission of light from excited hydrogen gas. The gas emits light only at specific frequencies rather than a continuous range of frequencies. Explain why excited hydrogen atoms emit light only at specific frequencies, and what this tells us about the energy levels inside an atom.

> AQA A-Level Physics (7408) — 3.2.2 EM Radiation and Quantum Phenomena · Explain · 5 marks

> When hydrogen gas is supplied with energy, it emits light at a small number of distinct frequencies. This produces a line spectrum rather than a continuous spectrum.

## Mark scheme (5 marks)

1. Electrons in an atom can only exist at specific, discrete energy levels (not between them)
2. When an electron falls from a higher energy level to a lower energy level, it releases energy as a photon
3. The energy of the photon equals the difference between the two energy levels
4. Because only certain energy level differences are possible, only certain photon energies (and therefore frequencies) can be emitted
5. This shows that energy inside the atom is quantised / energy levels are fixed properties of each element, explaining the unique line spectrum

## Key terms

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

## Related

- [Revision notes for AQA A-Level Physics (7408)](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-scientist-is-investigating-the-emission-00ce6f46) · Published by Druglandscape Ltd.