# Explain why the atmosphere emits infrared radiation back towards the Earth's surface, and outline why this re-emission occurs over a range of frequencies rather than at a single frequency.

> IB DP Physics Higher Level (2023 syllabus) — B.2 Greenhouse effect · Explain · 4 marks

> Greenhouse gases in the atmosphere absorb outgoing infrared radiation from the Earth's surface and subsequently re-emit radiation in all directions, including downward towards the surface.

## Mark scheme (4 marks)

1. Greenhouse gas molecules absorb infrared photons, causing their vibrational (or rotational) energy levels to become excited / molecules are raised to higher quantised energy states.
2. When excited molecules return to lower energy states they re-emit photons / infrared radiation in random / all directions, including downward toward the Earth's surface.
3. The range of frequencies arises because the atmosphere contains molecules at a range of temperatures / a distribution of kinetic energies, so collisions cause molecules to emit with a spread of energies (linked to thermal / blackbody-like emission).
4. Additionally, different greenhouse gas species (e.g. water vapour, CO₂, methane) each have different vibrational modes / absorption frequencies, so collectively the atmosphere emits across a broad infrared spectrum.

## Key terms

- [infrared radiation](https://www.gradenine.co.uk/glossary/infrared-radiation)
- [re-emission](https://www.gradenine.co.uk/glossary/re-emission)

## 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/explain-why-the-atmosphere-emits-infrared-d6cd7785) · Published by Druglandscape Ltd.