Does the speed of light depend on mass density? Light is a non-mechanical wave. It means that it does not require a medium to travel. But the characteristics of the medium to affect light. That s why refraction occurs, right?
I also read somewhere that the mass density of a medium does not affect it's speed of light, but instead it's optical density does so.
What would a quantitative definition of optical density be? Why is it not related to mass density and how does it affect the speed of light?
Why does the presence of more mass in less volume (more mass density) not affect the speed of light?
Thanks!
 A: The thing “slowing down” light in a dielectric material is not the amount of material per unit volume necessarily but the interaction of the electric field component of the light wave with the electron cloud orbiting each atom.
In fact you get the same behaviour if you study the speed of sound in gasses at different pressures. Here pressure is analogous to mass density i.e. high pressure implies more mass per unit volume. What you see in this case is that it’s not the pressure that determines the speed of sound but how compressible the gas is. Moreover the ratio of specific heat capacities is what is controlling the wave propagation speed.
Let’s return to optics, what I’ll describe is basically the Drude model.
The electron cloud will have certain natural mode/motions and will be perturbed by the electric field of the light wave. 
This becomes something similar to forced oscillation problem. Moreover, the light wave is perturbing the electron cloud as its frequency $\omega$, if this is close to one of the cloud’s modes $\omega_{01}$ then propagation is slowed.

The reason for the change in propagation speed is actually quite complicated. The light wave forces the electronic cloud to oscillate at its frequency, but there will be lag in response. The accelerating and decelerating cloud will emit light in response which is out of phase with original and this has the effect of slowing down the propagation.
In summary, refractive index in solids is dominated by the interaction of the forcing light wave electric field and the electronic cloud. Not the number density of atoms.
