system
Given the following post, respond as if you're commenting on Reddit:
human
If refraction is caused by the absorption and reemission of photons, why don't materials only refract certain wavelengths of light?

It's my understanding that light seems to slow when entering a more optically dense medium because the photons are being reabsorbed and re-emitted by the atoms of the medium, which takes time and results in a lower average speed of light. 

However, don't atoms also only absorb photons of certain energies? Wouldn't this mean that only certain photons with the right frequency would be refracted and those with other frequencies would carry on straight through?
gpt
Refraction is an emergent property of a material, that is, it only arises when there is a large number of atoms that the light is passing through. For instance, if you have 5 carbon atoms, that group of carbon atoms will not have the refractive properties of a diamond which is made of carbon. 

So, how do large collections of atoms have continuous absorption patterns? Because large collections of atoms have many more absorption methods than simple electron promotion. Collections of atoms can vibrate and oscillate, using two to n atoms (where n is some large number) which allows a nearly continuous absorption pattern as they are many, many combinations.