Light: Refraction and Reflection Why does light sometimes refract and some times reflect? Is it possible to predict whether light will reflect or refract given two media in which light under goes interaction?
 A: I assume you are asking about visible light.
When a photon interacts with an atom, three things can happen:


*

*elastic scattering, the photon keeps its energy level and changes angle

*inelastic scattering, the photon keeps part of its energy and changes angle

*absorption, the photon gives all its energy to the atom
When light interacts with the material, all three happen, it is just the ratio of the three that is different. For metal, most of the photons get 1. elastically scattered (specular reflection), and only a few get inelastically scattered or absorbed. For other materials, like a wall, most photons get absorbed (and re-emitted), and only a few get elastically scattered (diffuse reflection) or inelastically scattered.
Now some materials, like glass, can do both reflection and refraction at the same time.
Now in the case of reflection and refraction (with glass), both are 1. elastic scattering. This is the only way to keep the energy level and phase and relative angle of the photons, and keep a mirror image.
Now what is the difference then between when photons reflect or refract from glass? It is the angle.
But why do some photon change angle backwards (back into the original medium), and why do some photons keep their incident angle and move in the same direction forwards?
It is the lattice structure of the glass. When the photons interact with the lattice structure of glass, the partial waves of the photons pass through the space between the atoms (like in the case of the double slit experiment), and then the partial waves interfere with each other.
This interference can be destructive, these will be the directions where the photons will not travel, and the interference can be constructive, that is the only direction where the photon will travel.
Now the photons actually move in all directions, but they cancel out in every direction except the one where the photon will actually travel. This will be the direction where the interference will be constructive. All other directions the interference will be destructive.
Now you can still ask why will some of the photons travel backwards into the original medium, and why do other photons travel into the new medium?
It is QM, it is all about probabilities, and the lattice structure. With glass, the lattice structure is so, that 96% of the photons will refract, and 4% will reflect. 
