Deviation in refraction related question Why in refraction incident ray remain undeviated when incident angle is 0°? Please give the molecular interpretation.
 A: First lets talk about light propagation in a normal (not birefringent) material made of molecules.
The incident light causes a time-varying induced polarization in the molecules.  The polarized molecules then generate radiation of their own.  This re-radiation has two effects.  First, it exactly cancels (by interference) the incident radiation.  Second, it produces the refracted wave.  This result is called the Ewald-Oseen theorem.  
The direction of propagation of the refracted beam is perpendicular to the planes of constant phase of polarization.  For a tilted incident beam, the planes of constant phase are also tilted.  For an incident beam having a $0{}^\circ$ angle of incident, the planes of constant phase of the polarization are perpendicular to $0{}^\circ$.  So the angle of refraction is also $0{}^\circ$.
A simpler, more powerful but perhaps less satisfying way to think about it is to ask:  if the angle of incidence is $0{}^\circ$, what azimuthal direction would the refracted beam go?  If the material is symmetrical, there is no preferred azimuthal direction.  The only direction that maintains the symmetry is: straight ahead.
A: Refraction is explained by the wave nature of light. What happens is that the incident light wave wavefront is slowed down by the optically denser material (glass slab, lens etc.). If the angle of incidence is more than 0 degrees, the parts of wavefront are slowed down as soon as they interact with the denser material but since the parts away from the material are still moving at a faster speed(original speed) , the light deviates from its path . In the case where the angle of incidence is 0 degrees, the light does slow down but it keeps moving at the same path as before and there is no deviation. 
This is kind of hard thing to explain in words, so I would suggest that you watch some videos on refraction. 
Why light slows down is because the way the light(Electromagnetic Radiation) interacts with the molecules of the material. I don't know much about how exactly it happens, but I can tell you that it has something to do with the electric field of the electromagnetic radiation and the charges inside the material(charges have their own electric fields). Search on Google if you are interested. 
I am sorry if I couldn't clear your doubt. 
