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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (4 marks)
Greenhouse gas molecules absorb infrared photons, raising their vibrational (or rotational) energy levels to higher quantised states. When the excited molecules return to lower energy states they re‑emit photons – infrared radiation – in random directions, so some of this radiation is directed downwards toward the Earth’s surface.
The re‑emitted radiation is not at a single frequency because the atmosphere contains molecules at a range of temperatures, giving a distribution of kinetic energies. Collisions between molecules broaden the energy levels, so the emitted photons have a spread of energies, producing a thermal or black‑body‑like spectrum. Moreover, different greenhouse gases (e.g. H₂O, CO₂, CH₄) have distinct vibrational modes and absorption frequencies, so together they emit across a wide range of infrared frequencies.
The re‑emitted radiation is not at a single frequency because the atmosphere contains molecules at a range of temperatures, giving a distribution of kinetic energies. Collisions between molecules broaden the energy levels, so the emitted photons have a spread of energies, producing a thermal or black‑body‑like spectrum. Moreover, different greenhouse gases (e.g. H₂O, CO₂, CH₄) have distinct vibrational modes and absorption frequencies, so together they emit across a wide range of infrared frequencies.
Examiner tips
- Use the word "absorb" and "re‑emit" to show the energy transfer process.
- Mention that emission is random in direction to justify downward radiation.
- Explain the spread of frequencies by citing temperature distribution and different gas species.
- Keep the answer concise – 4 marks, so one or two sentences per point.
Common mistakes
- Confusing absorption with scattering – students say the atmosphere simply reflects IR instead of re‑emitting.
- Failing to mention that emission is in all directions, not just downward.
- Giving a single frequency or ignoring the role of different gases in the spectrum.
Mark scheme (4 marks)
- Greenhouse gas molecules absorb infrared photons, causing their vibrational (or rotational) energy levels to become excited / molecules are raised to higher quantised energy states.
- 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.
- 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).
- 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 in this question
infrared radiation · re-emission
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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