# Why does light either get reflected, absorbed, or passes through a surface or obstacle?

For each of these 3 cases, I'm having trouble understanding...

• If light is reflected, does that mean that there was not sufficient energy by the photons to excite the electrons of the surface to their higher orbitals? If that's the case, why would blue light sometimes be reflected, but not lower energy frequencies such as red light?
• Why would light be reflected rather than absorbed? What causes the photon to "bounce backwards"?
• If light passes through, is a photon literally passing through from end to end of one side of the material to the other? If so, can another situation where light excites electrons to a conducting band and these electrons create wave of electrons that propagate to the other side of the material where photons are then emitted?

EDIT: feel free to include mentions of refraction, scattering, etc. if you'd like.

• You should also include refracted, scattered and diffracted!! Apr 12, 2023 at 21:23

On a general note: photons are not actually corpuscles. A rational definition of a photon would be "A photon is an irreversible energy, momentum and angular momentum transfer between an EM field and an external system.", which means that "photons" characterize absorption and emission processes. If you look at the history of quantum mechanics, then you will notice that the first time "quantized energy transfer" was postulated was for the Planck spectrum problem, which is an irreversible emission process. The second time it was an irreversible absorption process: the photoelectric effect. Eventually we learned to apply this to atoms, molecules, nuclei, solids etc..

How does classical (reversible!) reflection fit into this picture? Well, for a macroscopic mirror the total momentum and angular momentum transfer and the resulting energy loss between the absorption of an "incoming" photon and the emission of another, slightly less energetic "outgoing" photon are negligible. We basically treat the two amounts of energy as "the same", even though they are not (and they would not be the same in the classical wave picture, either, if we weren't assuming infinite mirror mass). Since no (measurable) amount of energy is lost, the actually irreversible process appears reversible.

A similar argument can be made for light passing through a medium without being absorbed: whatever may happen microscopically inside the medium, we can treat it as a black box that absorbs one photon and emits another photon with the same energy but possibly with a different momentum (classically this is a change in wave vector or the change in the angle of the light beam) and angular momentum (for polarizing materials).

At the microscopic level the absorbed photon energy will excite the internal degrees of freedom of these optical materials. While no electrons will be "raised" to higher energy levels like they would in an atomic absorption process, the many-body wavefunction of all electrons in the solid will be excited. This excitation will be coherent, i.e. it will retain the statistical distribution between many photons that cause the classical electromagnetic field to exhibit wave properties.

Does this have to be so? No. It does not. Transparent and reflective materials form a special class of materials which happen to keep light coherent. There are many other materials that do not do this. Some show fluorescence and phosphorescence, for instance, i.e. they absorb photons of one energy and emit photons of another. For high enough photon flux many materials can exhibit non-linear mixing effects, i.e. they emit the sum and difference between photon energies or the sum and difference between the incoming photon energy and their internal vibrational energies. Semiconductors can absorb photons and cause electron/hole pairs to move or they can recombine electron hole pairs to emit photons. We know many other phenomena of this kind that happen when electromagnetic radiation interacts with matter. The details are generally complicated and material dependent. As a physicist you could spend a lifetime on researching the details of these processes and many people have.

• Regarding coherence, is it correct to say that a blue material keeps the blue visible light coherent? And that means the excitation happens in a way that forms a electromagnetic wave resulting in the reflected blue light? Apr 12, 2023 at 20:15
• Transparent materials keep light coherent. Non-transparent materials do not. Those will change the direction of the light more or less randomly. A blue material will reflect blue light but absorb light of other colors. Apr 12, 2023 at 20:48
• I think I understand. I conflated reflection with non-transparency, but I think I understand it as 1. coherent: transparent or reflection/mirroring, 2. de-coherent: all else typically. I'll need to reread your reply and look into this much more. Thank you for your answer. Apr 12, 2023 at 23:47

Light is many photons, AND light is a wave. Electrons can only hold the energy of a photon if that photon raises the electron to another energy level in that atom, a level that has an open space for that electron. Electrons can also absorb a photon with enough energy to free it from the atom. A photon does not have to interact with an atom just because the atom is there. A photon can pass through an atom.

Each photon can leave the atom at any angle. Which is most likely can depend on how long the atom holds the energy. If the photon energy cannot be held long, then the released photon is more likely to continue forward with very little change of momentum or kinetic energy. If it can be held longer, then it has more likelihood to come out in a new direction. If the photon matches the electron energy change very well, then it can remain in the atom and end up distributed through the material as kinetic energy. This is an increase of temperature. If temperature rises enough, you may start the material on fire.

With a narrow beam of light, with very many synchronized photons, some angles will result in coherent waves. These are usually the angle of reflection and the angle of refraction. The refraction angle can depend on the average time a photon is held by an atom and on the average number of atoms that a photon actually interacts with along its path through the material. These delays reduce the average speed of the photon. This determines the index of refraction. Some of the light reflects from the material and some light passes through.

• What determines the ability of an atom to hold the incoming photon's energy? e.g. why can't atoms in silicon dioxide (glass) hold onto photons for very long? That electrons are arranged where there is a "lack of space" such that it would require more energy than visible light? Apr 12, 2023 at 20:11
• This is quantization. An electron has specific states at which it can exist within an atom. Each has its own energy. Only one electron can exist in each state (two if you don't count spin as part of the state itself). When an electron rises from one energy state to another, it does not slowly increase in energy. It is in the lower energy state, and then it is in the higher energy state. Physical space is not a factor. Apr 13, 2023 at 20:32