Lock and key

A fit allows absorption

Use the right key and the lock opens. Fire the right photons at the right kind of matter and interesting things happen. Some colours trigger an effect: others do not. Photosynthesis, the colour of flowers, walls and bicycles are all determined by what between photons and the chemicals in the materials.

An inexact fit: resonance

Many locks work with keys even if the fit is a bit loose. That’s now it is with matter and photons: sometimes the fit is exact, sometimes not.