Why does the speed of light change? While researching the cause of refraction, I found that refraction occurs due to the change of speed of light when it goes from one medium to another (according to Huygens principle).
But I cannot get why actually does its speed change when the medium changes?
 A: The speed of light in a medium is given by $\frac{1}{\sqrt{\epsilon \mu}}$, where $\epsilon$ and $\mu$ are the relative permittivity and permeability of the medium respectively (these in turn affect how large the electric & magnetic forces in the medium are).
Since the permittivity & permeability of materials vary, it's not surprising that the speed of light varies as well.
A: As Wikipedia’s article “Refraction” explains,

Light slows as it travels through a medium other than vacuum (such as air, glass or water). This is not because of scattering or absorption. Rather it is because, as an electromagnetic oscillation, light itself causes other electrically charged particles such as electrons, to oscillate. The oscillating electrons emit their own electromagnetic waves which interact with the original light. The resulting “combined” wave has wave packets that pass an observer at a slower rate. The light has effectively been slowed down. When light returns to a vacuum and there are no electrons nearby, this slowing effect ends and its speed returns to $c$.

