hi .. light  moves slowly in a medium because it travels unhampered with constant speed 3*108 m/s ,while through any other medium light scattered as a result of collisions with atoms and molecules of that medium. 

http://www.rpi.edu/dept/phys/Dept2/APPhys1/optics/optics/node4.html

I would add short comment that 'slowly' or 'faster' are not so descriptive for ligth velocity. We can use phase or group velocity terms to be more specific.

The answer depends on whether you mean in a dielectric of a metal. I'll assume you mean a dielectric, transparent material. Basically, the electromagnetic field of the light interacts with the atoms in the material (with the dipole moments or phonon field). This is not absorption and re-emission as many people say. To a photon, a solid is more like a liquid, i.e. the photon "feels" the electromagnetic behaviour of the atoms as a bulk, not as individuals. The result is a change in the wavelength of the light., required my Maxwell's equations). Then, due to the dispersion relation (c=freq x wavelength), the frequency must be conserved, so the speed must change. 

Arguing that the photon "feels"  the atoms as a liquid sounds fine when the medium has near solid density, or even gaseous density. It is not clear to me where you get the deducting in wavelength from. Certainly the frequency is fixed and the wavelength and velocity have to vary together. 

Consider starlight crossing the voids where the particle density is ~1/m^3. This still slows the light, that's why neutrinos from super nova arrive early. Yet it's hard to argue that this is a fluid. The particles are many wavelengths apart. The particles certainly load the wave. So how is loading of the wave any different from absorption and re-emmision.

If the various models all give the same answer you can't argue that any one is more valid than any other. There is no fundamental truth. There is only a model that gives useful answers.

Yes, there are flaws in my simplification. But the QED model of em motion through a solid (the best we have) does not require absorption and re-emission. Dielectrics have no absorption. The slowing (or the wavelength change) is cause by an interaction with dipolar atomic field which causes a phase change (it is a resonant phenomena).

So is that the wave loading I mentioned? That would certainly lead to a phase shift. If so a single particle in free space will move in response to the wave/photon. If there is no attenuation of the wave the particle must end up with no net energy change, yet in the process it is accelerated, so it has absorbed energy for a while. How is this not absorption and re-emission. Perhaps it is scattering and inverse scattering. Is there a real difference.

Sorry for delay, I'm having internet issues here in China. The difference is subtle. The presence of atoms (however dilute) causes changes in the permitivity and permeability, which are related to the speed/wavelength (as f constant).  For transparent dielectric materials, there is a linear relationship between the net electric dipole moment per unit volume (also called “electric polarization”). The electric field induced by an oscillating dipole moment is in-phase with the electric dipole, and implicitly, oscillates in-phase with the external electric field. Therefore, the radiation released by an oscillating dipole is necessarily in-phase with the incident light. That's why it doesn't change frequency. But, by due to scattering, the light loses energy laterally when it travels through the dielectric. But scattering is not the same a absorption - they are fundamentally different processes.