If I understand correctly, when photons encounter a boundary between media, some of them are reflected/scattered back, while some are transmitted/refracted through to the other side. What determines how many photons go back and how many go forward? I understand Fresnel equations describe the behavior, and are based on the refractive indices of the media. But then what determines the refractive index of a medium? For example, what is it about water that refracts most visible light through, while white paint reflects/scatteres most of it back at us? Can someone please give me an explanation of what is happening at the level of the oscillating electrons inside the atoms/molecules?