Why does light reflect? Can anyone please explain why light reflects at the boundary between two regions with different impedances? This sounds very simple but I got confused when I tried to think of how light and atoms interact with one another at the boundary.
This question can actually be generalised to the reflection of all types of waves. I have to admit I have no understanding of the microscopic detail of reflection.
 A: The way I would recommend you approach this question is to look at the derivation of Fresnel's Laws of Reflection, as is shown in detail at this link. The key here is that there must be a continuity of the electric field across the boundary between the regions of different refractive index - but since such regions have different amounts of polarization for the same electric field (that is pretty much the definition of relative dielectric constant...) the only way that this can happen is if part of the electric field "stays outside" while another part penetrates into the second medium. Which is what we call "partial reflection".
A: In addition to all that has been said, maybe the following will help the intuition:  The electrons around the atoms at the surface of the glass is set in vibration by the oscillating electric field of the light beam.  But a vibrating electric charge will itself send out an electromagnetic wave.  This secondary wave generated at the surface will propagate not only into the glass, but also into the air and give rise to the reflected beam.    
