# How does absorption of light by atoms and molecules work?

I found this question on a hobby science forum (mainly about chemistry) and found embarrassingly that I couldn't answer the question. A few searches along the lines of 'photons absorption' here on Ph.SE yielded surprisingly little.

Does anyone here have anything near a comprehensive answer?

• When you say absorption do you mean this in the sense of an absorption line for a gas or in the sense of something being opaque. There's a big difference because in the former process light is absorbed then re-emitted in a different direction while in the latter case the light is converted to heat. – John Rennie Mar 1 '16 at 16:13
• @JohnRennie: it appears the OP means it in the sense of something being opaque, like coloured glass. – Gert Mar 1 '16 at 16:22

This answer assumes that absorption of light means the conversion of light to a different form of energy.

If you start with an isolated atom or molecule, the a photon of the right frequency can be absorbed by exciting the atom/molecule to a higher energy state. Why this happens is discussed in my answer to How do photons know they can or can't excite electrons in atoms?. The excited state will decay back to the ground state and re-emit a photon. So the light is not lost though it may be scattered into a different direction.

In a dense medium like a liquid or solid the atoms/molecules interact mechanically with their neighbours i.e. vibrations of the atom/molecule can be transmitted to neighbouring atoms/molecules and vice versa. In these conditions it is very likely that the excited atom/molecule will transfer its extra energy into mechanical vibrations before it has a chance to re-emit the energy as light. The end result is that the light is turned into vibrational energy i.e. heat.

In a comment the OP mentions coloured glass as an example of what is meant by absorption. If we take for example green glass, this is frequently coloured using iron (II) salts. Light is absorbed and promotes electrons between the $\text{Fe}^{2+}$ $3d$ orbitals (the $3d$ orbitals are split into different energies by the environment). The excited electrons relax by transferring energy to the surrounding chemical bonds as vibrational excitations, which turns the absorbed light into heat.

The real answer is " quantum magic." Otherwise, it's simply an observed effect that a photon will be absorbed when its energy matches the transition energy of an electron (said electron gaining potential energy).

If you move to very high-energy photons which are absorbed in the nucleus, the rules get messier but the basics are the same: the particles in the nucleus have their own set of energy states (or energy of fission) which have to match the photon energy. That's very top level, of course.

• There is a little more to it than just energy intervals. The angular momentum quantum numbers of the involved states have a lot to say about the probability of absorption. There are so called allowed or forbidden transitions due to these effects. – Lewis Miller Mar 1 '16 at 16:11