TITLE: What exactly happens during the reflection of light at the atomic level?
QUESTION [0 upvotes]: I studied in Ray Optics that light either reflects, refracts or gets absorbed when it encounters another medium. But what exactly happens at the atomic level during reflection, refraction and absorption? At the macroscopic level, we see that light is reflecting, refracting or getting absorbed. But what happens at the atomic level when these phenomenan occur? Also, how can we decide whether light will reflect, refract or get absorbed when it encounters another medium. When exactly do these phenomenan take place? Please answer these two questions. I have been having these question since Grade 7 and now I am in Grade 12. I am so confused. Would greatly appreciate if someone could answer my above two questions.

REPLY [1 votes]: What exactly happens during the reflection of light at the atomic level?

Atoms and molecules are quantum mechanical entities, i.e.  their interactions are controlled by quantum mechanical solutions of the appropriate equations for the particular problem. Reflection, refraction and absorption are well described by the classical Maxwell equation solutions and the simplification of ray optics. Nevertheless the classical behavior is a coherent build up from photon interactions, photon + atom, photon+molecule, photon +lattice interactions .
A a single photon interacting with a quantum particle can either,

elastically scatter. This means that the light wave it is part of does not change frequency/color.

scatter inelatically  part of its momentum/energy is transferred to the atom/molecule latice and its color can change

be completely absorbed by the atom/molecule/latice because it energy was appropriate for changing the energy levels in the atom/molecule/latice and then different phenomenacan be induced: luminenscens, opaqueness, ...


The mathematical prediction fora single photon is a probabilistic one, with Ψ the wavefunction describing the photon and the probability of reflecting or.. given by $Ψ^*Ψ$.
The light beam , a quantum mechanical superposition of a great number of photon, after interacting with atoms/molecules/lattice depending on the surfaces can be coherent, thus images can be reflected and if the scattering in a lattice is coherent then images can also be transmitted, as with transparent materials.

Also, how can we decide whether light will reflect, refract or get absorbed when it encounters another medium.

The "decision" is given by the macroscopic measured indices of the material, for example. , There are too many variables in the quantum mechanical equations to be able to predict from the quantum frame of individual particles to the macroscopic behavior of light.
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