Greenhouse gases absorb particular wavelengths of outgoing infrared radiation when the radiation's frequency matches an allowed molecular energy transition. The excited molecules then transfer energy through collisions or re-emit infrared radiation in all directions, reducing Earth's net rate of energy loss to space.
This is part of B.2 Greenhouse effect and is studied by both SL and HL students.
Earth's surface absorbs incoming solar radiation and warms. Because Earth is much cooler than the Sun, it emits mainly longer-wavelength infrared radiation rather than visible light.
Molecules have quantized rotational and vibrational energy levels. Greenhouse gases such as , , , and can absorb infrared photons whose energies match differences between these levels:
The absorbed energy increases the molecule's rotational or vibrational energy. In the IB resonance model, absorption is described as occurring when the frequency of the infrared radiation matches a natural frequency of molecular vibration.
An excited molecule may collide with surrounding molecules, transferring energy to their random motion and increasing the gas's internal energy. Alternatively, it may return to a lower energy level and emit an infrared photon. This radiation is emitted in random directions: some travels toward space, while some travels back toward Earth's surface.
The downward component is absorbed by the surface and lower atmosphere. To restore energy balance between incoming and outgoing radiation, Earth's surface reaches a higher equilibrium temperature than it would without greenhouse gases.
A common misconception is that greenhouse gases mainly block incoming solar radiation. In fact, they are largely transparent to incoming short-wavelength radiation but absorb selected bands of Earth's outgoing infrared radiation.
Exam technique: For an Explain question, link molecular energy levels to wavelength-specific absorption, state that radiation is re-emitted in all directions, and connect the downward radiation to a higher equilibrium surface temperature. Do not say infrared radiation is permanently “trapped.”
