Molecules and Light

How molecules absorb and emit electromagnetic radiation

Molecules absorb electromagnetic radiation when the photon energy matches a transition in the molecule — vibrational transitions for IR, electronic transitions for UV/visible. CO₂ and H₂O absorb IR radiation (heat) because their bending and stretching modes match IR photon energies — the greenhouse effect. N₂ and O₂ are IR-transparent but UV-absorbing at higher energies. Ozone absorbs UV-B and UV-C, protecting life on Earth.