Main Question or Discussion Point
what happens with the atoms, while there is reflection of light? What happens with the electrons or protons, are they reflecting the light and how?
I understand that if the energy of the photon equals to the gap between two energy levels in the atom - the photon will be absorbed. if the energy of the photon is not equal to the energy gap - why in some of the cases the photon will be reflected and in some of cases it will pass (transparent substrate) ? what causes it ?Light interacts with atomic electrons, not the nucleus where the protons and neutrons reside.
The photons are simply redirected (scattered or reflected) and the atom remains stationary (more or less) since the momentum of the photon is very small and the mass of the atom is very large.
Can you please explain how the electrons reflect the light-how does this electron-electromagnetic radiation interaction working ? why in transparent substrates it doesn't "work"?Take note that "reflection of light" that most people are familiar with occurs with a typical mirror, i.e. a metallic surface. This is a "solid", and also a conductor. Lesson to learn when something has formed into a solid:
1. The "atoms" no longer play a major role in the bulk behavior of a solid. It has lost most of its individual identity (example: graphite and diamond. Both made up of carbon atoms, yet they each have such different properties as a solid). So when we are dealing with a solid, we need to consider the collective behavior, not the properties of individual atoms. (I know I've said this a million times already, if not more. Read the FAQ related to light transport in a medium.)
2. A metal or conductor has bands, and in this case, what is relevant is the conduction band. These "free" electrons plays the most important role in the reflection of light (at least in the visible range).
wikipedia said:The law of reflection arises from diffraction of a plane wave (with small wavelength) on a flat boundary: when the boundary size is much larger than the wavelength then electrons of the boundary are seen oscillating exactly in phase only from one direction — the specular direction. If a mirror becomes very small (comparable to the wavelength), the law of reflection no longer holds and the behaviour of light is more complicated.