Simple Publishing
đź““ Reflections
Why do objects reflect light? If atoms are mostly empty space, why does light not pass through it? If it does not pass through, why is it not absorbed?
The answers here, all sincere attempts, seem to be missing the explanation of mirrors, color, and other common material properties. These are best understood at the level of the electromagnetic field, rather than quantum properties.
Light is an electromagnetic wave.
Matter consists of charged particles which are held in place by electrostatic fields (roughly, I realize the electrons are actually moving, though the nuclei can be treated as mostly stationary and the electrons at “average” positions). Any of these charged items (electrons, nuclei) can absorb and re-radiate an EM wave, just like electrons in a conductor.
Thus matter looks kind of like a giant antenna. Have you ever seen an old fashioned TV antenna with a bunch of little aluminum rods at mysterious spacings? These correspond to the atomic or molecular spacing. The difference is TV antennas operate at relatively long wavelengths, and optical media at shorter wavelengths, thus shorter spacings.
if the spacings are such that the incident wave sees a low impedance, that is,Ă‚ if the resonance due to the spacings is such that no EM potential arises on the surface, then effectively a surface current is flowing which is out-of-phase and re-radiates the incident wave back the way it came. If this re-radiation is coherent, we get a mirror. Otherwise, we just get a bright object, e.g. white.
For mirrors, there are two further qualities of interest.Ă‚ The electrons in the material need to be mobile, to suppress any EM field at the surface over a wider range of wavelengths. Thus most mirrors are based on metal. Second,Ă‚ the surface needs to be optically flat, so that the return wave is coherent. Thus mirrors must be highly polished.Ă‚ Practical mirrors involve deposition of metal onto a piece of glass rather than polishing the metal directly, because it is easy to make very flat glass.
If the surface current is partly in-phase, because the resonances are not tuned to the incident light, then this constitutes a real impedance which absorbs some of the incident radiation as heat, and the color of the material will be darker.
Some materials are more narrowly tuned than others, and may reflect some frequencies and not others, having an appearance other than gray or white.
Without solving Maxwell’s equations, which is very difficult for an array of elements like a material surface and at best done computationally, not analytically, that is about the best I can do. It is possible someone else will be able to carry the antenna-analogy further and give some rules of thumb. If so, please leave a note here so that I will notice it.