Why does CO₂ absorb IR but not visible light?

CO₂ vibrational mode energies match IR photons (~0.1 eV); visible photons (~2 eV) can't excite these modes.

Why are N₂ and O₂ transparent to infrared?

Symmetric diatomic molecules have no net dipole change during vibration — can't couple to IR field.

How does a microwave oven work? What molecular motion is excited?

Microwaves match water rotational transitions (~10⁻³ eV), causing rapid rotation and friction heating.

Pro 🔒~20 min

Molecules and Light

How molecules absorb and emit electromagnetic radiation

How it works

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.

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Step-by-step

Select a molecule from the menu.
Choose a radiation type.
If the molecule absorbs that radiation, watch it vibrate or rotate.
The energy level diagram shows which transitions are available.
Explore CO₂ + infrared to see the greenhouse effect in action.

Key formulas

$E = hf = \frac{hc}{\lambda}$ Photon energy
$\Delta E_{vib} \approx 0.1\text{–}1\text{ eV (IR)}$ Molecular vibration energy scale

Frequently asked questions

Why does CO₂ absorb IR but not visible light?: CO₂ vibrational mode energies match IR photons (~0.1 eV); visible photons (~2 eV) can't excite these modes.
Why are N₂ and O₂ transparent to infrared?: Symmetric diatomic molecules have no net dipole change during vibration — can't couple to IR field.
How does a microwave oven work? What molecular motion is excited?: Microwaves match water rotational transitions (~10⁻³ eV), causing rapid rotation and friction heating.