Photons and selective absorption

Photons used to explain the interaction of light with matter

Photons are all-or-nothing entities: you cannot emit or absorb half a photon. (Remember that to detect a photon you have to destroy that photon.) On destruction, a photon shifts all of the energy to a store, filling that store by a tiny and characteristic amount.

The power switched from the lighting pathway depends on the number of photons destroyed each second and the energy shifted by each. You cannot half-destroy a photon – it is all or nothing. Thus there are discrete micro-steps in the accumulation of energy in the store. This graininess led to the quantum name. The steps for blue photons are larger than the steps for red photons.

Inside materials, considering individual atoms and molecules, there are similar energetic steps: the stores can only be filled by discrete increments. Where there is a close enough match between the energy shifted by the photon and the energy the store can accept, the photon is destroyed and a process within the material is enabled. This explains processes like filtering by frequency: this is just filtering by energy, as the energy depends on the frequency.

In other cases, what is happening is different, and there is a threshold effect. So long as the photon provides enough energy, the process is enabled, and the excess is dissipated in some way.

Two kinds of absorption of photons

A well-rehearsed example of the threshold effect, which is important historically, is the photo-electric effect, where photons eject electrons from metals. Once there is enough energy to eject the electron, any extra energy turns up in the kinetic store of the ejected electron.

If there is no such dissipative mechanism, providing a store where the excess energy shifted by the photon can be dissipated, then only photons with a narrow band of energies will be absorbed. This is often called a resonant effect, and is much studied in further physics. Perhaps the simplest example, and so well worn, is that of a child on a swing. If you match the frequency of pushes to the natural frequency of the swing, you can easily augment the energy in the vibrational store. Pushing with the wrong frequency can be painful and does not facilitate the energy being shifted.

Resonant matching also underpins much of the information we have about the universe. By seeing which photons are absorbed on their way to us, you can work out what lies between Earth and the source.

Later you'll also see how emission also depends on this matching process, so allowing you to infer rather complete descriptions of the atoms and molecules literally light-years away.

Closer to home, plants are green because of the combination of photons that they reject by reflection.